Maintaining Electric Reliability with Wind and Solar Sources: Background and Issues for Congress
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Maintaining Electric Reliability with Wind
and Solar Sources: Background and Issues for
Congress
Updated August 4, 2022
Congressional Research Service
https://crsreports.congress.gov
R45764
SUMMARY
Maintaining Electric Reliability with Wind and
Solar Sources: Background and Issues for
Congress
R45764
August 4, 2022
Ashley J. Lawson
Analyst in Energy Policy
The share of wind and solar power in the U.S. electricity mix grew from 1% in 2008 to 13% in
2021. Wind and solar are variable renewable energy (VRE) sources. Unlike conventional
sources, weather variability creates uncertainty about the availability of VRE sources. This
uncertainty could potentially result in a lack of reliability.
Some Members of Congress have expressed concerns about the reliability of the electric power system given recent growth in
generation from wind and solar sources and projections that growth will continue. Generation from wind and solar sources
does not appear to be causing widespread electric reliability issues at the national level, though some solar generators have
exacerbated regional events in some cases. Questions remain, however, about maintaining reliability if generation from wind
and solar should increase above current projections, as some Members of Congress have supported. Entities in the electric
power sector and their regulators are evaluating changes to their approaches to reliability to prepare for this possibility.
Congress might explore whether new or modified approaches are required.
Under the current regulatory framework, the federal government oversees reliability for the generation and transmission
systems of the electric power sector. These components comprise the bulk power system and include large-scale wind and
solar sources. The Energy Policy Act of 2005 (EPACT05; P.L. 109-58) authorized the Federal Energy Regulatory
Commission (FERC) and its certified electric reliability organization, the North American Electric Reliability Corporation
(NERC), to develop and enforce mandatory reliability standards for the bulk power system. Small-scale wind and solar
sources, such as rooftop solar photovoltaic (PV) panels, are connected to the distribution system which is localized and under
state jurisdiction. Federal mandatory reliability standards do not apply to the distribution system.
A colloquial definition of reliability is “having power when it is needed,” but regulators and operators of power system
components require a more precise statement of objectives and metrics. FERC and NERC have developed numerous
technical standards to address reliability. These standards apply over the range of timescales over which reliability is
measured, from milliseconds to years. FERC has approved approximately 100 reliability standards to date, and new standards
are developed as needed to respond to changing conditions, including increasing generation from wind and solar sources.
Multiple entities spanning multiple jurisdictions work together to maintain electric reliability.
For economic reasons, wind and solar sources tend to be utilized to the maximum extent possible. When their availability
changes, which can happen quickly, other sources must quickly respond to maintain reliability. Typically, other sources
respond by increasing or decreasing their output, an operation known as balancing. Multiple types of electricity sources are
used to balance wind and solar, including some fossil fuel-fired generators, some nuclear generators, other renewable energy
sources (provided sufficient transmission availability), energy storage, and demand response. Each of these has benefits and
limitations. In the Infrastructure Investment and Jobs Act (IIJA; P.L. 117-58), Congress provided some support to electric
transmission infrastructure and certain energy sources, both of which could potentially promote electricity reliability. At the
time of publication of this report, implementation of these provisions is in the early stages, and any eventual impact of IIJA
on reliability is unknown.
Beyond developing and enforcing reliability standards, other federal government activities affect electric reliability. For
example, FERC’s regulation of interstate electricity transmission can be a key determinant of how effectively different
electricity sources can meet demand. FERC’s regulation of the wholesale electricity markets that operate in some regions of
the country also may affect reliability, because market rules can influence which individual generators are used for system
balancing. Market prices directly affect project revenues, influencing the kinds of sources that are developed. Additionally,
some projects and programs Congress funds support reliability by enabling technology development and providing financial
support for projects that support reliability. Congress also conducts oversight into electricity reliability, including power
outages caused by extreme weather, such as those affecting parts of Texas and other states in February 2021.
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Maintaining Electric Reliability with Wind and Solar Sources
Contents
Introduction ..................................................................................................................................... 1
Electric Power Sector Overview ..................................................................................................... 2
What Is Electric Reliability? ........................................................................................................... 4
Changing Electricity Generation Profile ......................................................................................... 7
Balancing Variable Renewable Energy ........................................................................................... 9
Federal Government Activities Affecting Reliability and Balancing ............................................ 12
The Infrastructure Investment and Jobs Act (P.L. 117-58) ...................................................... 14
Potential Issues for Congress......................................................................................................... 15
Figures
Figure 1. Simplified Schematic of Electric Power Sector Systems ................................................. 2
Figure 2. Map of Regional Transmission Organizations (RTOs) .................................................... 3
Figure 3. Generation Patterns for U.S. Wind and Solar Sources ..................................................... 7
Figure 4. Annual Net U.S. Electricity Generation by Source, 2008-2021....................................... 8
Figure A-1. Power System Reliability Timescales ........................................................................ 18
Figure A-2. NERC Regional Entities ............................................................................................ 21
Tables
Table A-1. Selected Entities Involved in Power System Reliability ............................................. 20
Appendixes
Appendix. Key Reliability Concepts for Policymakers................................................................. 18
Contacts
Author Information........................................................................................................................ 22
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Maintaining Electric Reliability with Wind and Solar Sources
Introduction
Economic factors, new technologies, aging of power plants, climate change concerns and
associated regulatory policies, among other developments, are changing the energy sources used
to generate electricity in the United States. One notable change is increased generation from
variable renewable energy (VRE) sources such as wind and solar. According to the U.S. Energy
Information Administration (EIA), combined generation from wind and solar sources increased
from 1% of total electricity generation in 2008 to 13% of total electricity generation in 2021.1
These sources typically have time of day and weather-dependent availability, meaning that
changing weather patterns can change available electricity supply from those sources. In contrast,
conventional “base load” sources for electricity generation, such as coal, natural gas, or nuclear
energy, are usually available under normal weather conditions.2 Power system operators have
adjusted existing reliability standards and planning practices to accommodate weather-dependent
wind and solar sources. Further adjustments are being discussed by regulators and market
participants as generation from wind and solar sources continue to grow.
Congress required the setting and enforcement of electric reliability standards in the Energy
Policy Act of 2005 (EPACT05; P.L. 109-58).3 These standards are developed by the North
American Electric Reliability Corporation (NERC) and approved by the Federal Energy
Regulatory Commission (FERC) in the United States.4 These mandatory standards apply to the
bulk power system, which is comprised mostly of large-scale generators and electricity
transmission systems. Distributed generators (e.g., rooftop solar electricity generation), publicly
owned utilities, and local electricity distribution systems are generally under the jurisdiction of
state public utility regulatory commissions (PUCs).
To date, generation from wind and solar sources does not appear to be causing widespread
electric reliability issues at the national level, though the devices handling the form of electric
current typically produced by solar generators exacerbated regional events in some cases.5
Questions remain about how anticipated higher levels of generation from wind and solar sources
might affect electric reliability moving forward.
1 CRS analysis of data from U.S. Energy Information Administration (EIA), “Electricity Data Browser,” accessed May
19, 2022, https://www.eia.gov/electricity/data/browser/. EIA began collecting data on generation from small-scale solar
photovoltaic (PV) sources in 2014. The statistic reported for 2021 includes both large-scale and small-scale solar
sources. Typically, small-scale solar PV sources are connected to the distribution system, not the bulk power system.
2 EIA defines a base load plant as one “usually housing high-efficiency steam-electric units, which is normally operated
to take all or part of the minimum load [demand] of a system, and which consequently produces electricity at an
essentially constant rate and runs continuously.” EIA, “Glossary,” accessed July 25, 2022. Some renewable energy
sources, such as hydropower and geothermal energy, can power base load plants. Pairing variable renewable energy
(VRE) sources with sufficient energy storage might allow the resulting hybrid systems to behave like base load plants.
3 16 U.S.C. §824o.
4 Due to the integrated nature of the bulk power system, the North American Electric Reliability Corporation (NERC)
oversees electric reliability for parts of Canada and Mexico as well. The electric industry originally established
NERC’s predecessor, the National Electric Reliability Council, in 1968 to coordinate voluntarily reliability efforts.
Alaska and Hawaii are exempt from NERC’s jurisdiction.
5 For example, during events in California in 2016 and 2021 and separate events in Texas in 2021, some solar
generators went offline when grid disturbances occurred. The loss of electricity from these generators exacerbated the
underlying event, “causing a minor system disturbance to become a major disturbance.” NERC 2022 Summer
Reliability Assessment, May 2022, p. 6. Regulators have attributed this behavior to the way inverters—electronic
devices that convert the form of electricity generated by solar photovoltaics into the form used throughout the
electricity system—are programmed to operate. Industry participants and their regulators continue work to address the
issue.
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This report provides background on reliability planning in the United States with an emphasis on
the effects of daily and seasonal variability in wind and solar sources on the bulk power system.
Members of Congress might consider how reliability could be impacted if generation from wind
and solar sources increases, as many analysts expect. Other reliability concerns, such as cyber and
physical security, extreme weather events, distributed generation, and local distribution networks,
may be of interest to Congress but are not discussed at length in this report.
Electric Power Sector Overview
As shown in Figure 1, the provision of electric power consists primarily of three systems. The
generation system consists of power plants that generate electricity. The transmission system
generally consists of high voltage transmission lines that move power across long distances. The
distribution systems make final delivery of electricity to homes and businesses. This report will
refer to the combined generation and transmission systems as the bulk power system, following
the definition Congress established in EPACT05:
The term “bulk-power system” means—(a) facilities and control systems necessary for
operating an interconnected electric energy transmission network (or any portion thereof);
and
(b) electric energy from generation facilities needed to maintain transmission system
reliability.
The term does not include facilities used in the local distribution of electric energy. 6
Notably, the discussion in this report generally excludes distributed energy resources such as
rooftop solar electricity generation. These resources might pose separate reliability challenges
that Congress might choose to consider.
Figure 1. Simplified Schematic of Electric Power Sector Systems
Source: CRS, adapted from U.S.-Canada Power System Outage Task Force, Final Report on the August 14, 2003,
Blackout in the United States and Canada: Causes and Recommendations, April 2004, p. 5, https://www.energy.gov/
sites/prod/files/oeprod/DocumentsandMedia/BlackoutFinal-Web.pdf.
Note: Not all types of components in each system are shown.
6 16 U.S.C. §824o
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Ownership structures for bulk power system components vary across the country. In some
regions, shown in Figure 2, competitive markets exist for wholesale electric power, and regional
transmission organizations (RTOs) and independent system operators (ISOs) manage the
generation and transmission components of the power system. RTO regions generally underwent
“restructuring” of their electric utility companies. The utility companies created separate,
competitive companies for each power plant, with transmission remaining owned by the utility
company but managed by the RTO. Separate distribution companies were formed to sell power to
retail customers.7 Thus, in RTO regions, electricity generators compete to sell power to
distribution utilities. The RTO manages an auction process to select the sources for generation
producing the electric power that distribution utilities resell to end-use customers. The RTO also
is responsible for managing the transmission system and overseeing reliability within its
boundaries. In RTO regions, market signals primarily determine new power plant investment
decisions. Some RTOs operate separate auction processes specifically for essential reliability
services.8 According to FERC, two-thirds of U.S. electricity demand comes from RTO regions.9
Figure 2. Map of Regional Transmission Organizations (RTOs)
Source: FERC, “RTOs and ISOs,” https://www.ferc.gov/power-sales-and-markets/rtos-and-isos.
Notes: RTOs manage almost two-thirds of U.S. electricity generation, including day-to-day reliability
responsibilities. In non-RTO regions, shown as white in this map, vertically-integrated utilities, municipal utilities,
or electric co-operatives have these functions. ISO = Independent System Operator. Alaska and Hawaii are not
shown because RTOs and ISOs do not operate in those states.
7 A more detailed history of Regional Transmission Organizations (RTOs) and Independent System Operators (ISOs),
and further description of the functions summarized in this paragraph, are in CRS Report R44783, The Federal Power
Act (FPA) and Electricity Markets, by Richard J. Campbell, and CRS Report R43093, Electricity Markets—Recent
Issues in Market Structure and Energy Trading, by Richard J. Campbell.
8 The term essential reliability services refers to a group of functions that generators perform, usually automatically,
over timescales of seconds or less. Damage to bulk power system components could occur if sufficient essential
reliability services are not available. More details are provided in the Appendix.
9 Federal Energy Regulatory Commission (FERC), Electric Power Markets: National Overview, updated July 20, 2021,
https://www.ferc.gov/electric-power-markets.
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In non-RTO regions, vertically integrated electric utilities are largely responsible for power
generation, transmission, and distribution of electricity to end-use customers. These utilities are
essentially regulated as natural monopolies: under the “regulatory compact” concept, electric
utilities were granted near monopoly status in some states in exchange for agreeing to provide
universal service. Unlike utilities in RTO regions, vertically integrated electric utilities generally
do not face competition for generation and transmission services. These utilities also may take
responsibility for some aspects of reliability as discussed in the Appendix. State regulators
generally oversee these utility operations and are responsible for authorizing power plant and
transmission siting and allowing recovery of new investments in utility rates, including those
related to reliability.10
Even in RTO regions, municipal utilities and rural electric cooperatives may own generation and
transmission system components and provide regulatory oversight of their operations.11 These
systems and operations are generally outside of federal and state regulatory jurisdiction.
What Is Electric Reliability?
A colloquial definition of electric reliability is “having power when it is needed.” Operators of
bulk power system components, though, require specific and highly technical definitions for
reliability. For purposes of regulation, these definitions are provided in the form of NERC
reliability standards. NERC develops individual standards for each set of power system
components, which may include separate standards covering different reliability timescales for
each set of components. As NERC defines “reliability standard,” it
includes requirements for the operation of existing Bulk-Power System facilities, including
cybersecurity protection, and the design of planned additions or modifications to such
facilities to the extent necessary to provide for Reliable Operation of the Bulk-Power
System, but the term does not include any requirement to enlarge such facilities or to
construct new transmission capacity or generation capacity. 12
When all bulk power system components meet reliability standards, NERC expects the vast
majority of power customers to have the full amount of electricity they desire. NERC reliability
standards do not apply to local electricity distribution system components and operations (see
discussion in text box, “Distribution System Reliability”), so outages still could occur when
reliability standards are met. An analysis found that from 2008 to 2014, upwards of 90% of power
outages originated in local distribution systems.13 This measure includes major events (e.g., major
storms and hurricanes), but may not capture the full scope or severity of large-scale outages.
10 Federal Power Marketing Administrations are exceptions. For more details see CRS Report R45548, The Power
Marketing Administrations: Background and Current Issues, by Richard J. Campbell.
11 For further discussion of municipal utilities and rural electric cooperatives see U.S. Department of Energy (DOE),
United States Electricity Industry Primer, July 2015, https://www.energy.gov/sites/prod/files/2015/12/f28/unitedstates-electricity-industry-primer.pdf.
12 NERC, Glossary of Terms Used in NERC Reliability Standards, July 3, 2018, p. 26, https://www.nerc.com/pa/Stand/
Glossary%20of%20Terms/Glossary_of_Terms.pdf. This definition of reliability standard was established in the Energy
Policy Act of 2005 (P.L. 109-58) from which NERC derives its authority. See 16 U.S.C. §824o.
13 Researchers from the Institute of Electric and Electronics Engineers (IEEE) Distribution Reliability Working Group
and EIA found different values depending on whether outages are measured in terms of duration or frequency, whether
major events are included, and the methodology used for attributing an outage’s origination to either the bulk power
system or the distribution system. All measures of customer-weighted average values showed at least 90% of outages
originated in the distribution system. The mean and median values, however, sometimes showed a lower share (as low
as 83%) of outages originating in the distribution system. Joseph Eto et al., “Distribution System Versus Bulk Power
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Distribution System Reliability
As shown in Figure 1, the electric distribution system makes final delivery of electric power to businesses,
households, and other consumers. Voltage levels on distribution systems are lower than on transmission systems,
but otherwise many distribution system components operate in the same way as transmission systems. Different
ownership models for electric distribution utilities include investor-owned, publicly owned, and cooperative. In
some cases, the owner of distribution utilities also owns generation and transmission assets, but in other cases it
does not. Regardless of whether distribution utility owners also own and operate generation and transmission
system components, distribution utilities generally are regulated by state or local government entities.
Traditionally, reliability of most components of the distribution systems has not been viewed as a federal issue,
since the effects of outages in distribution systems tend to be localized. Nonetheless, distribution system outages
can be impactful.14 Past federal efforts to improve distribution system reliability include funding for Smart Grid
upgrades and grants to communities to install microgrids or other electricity projects.15
Small-scale wind and solar generators frequently are located on the distribution system. Often, these occur in socalled behind-the-meter configurations in which the generator is on the customer side of the distribution utility’s
meter. This is the case for rooftop solar panels, for example. These sources generally cannot be controlled by
system operators the way sources on the transmission system can be, raising potential reliability concerns. An
emerging proposal to account for this situation is an independent distribution system operator (IDSO) that could
balance generation and electricity demand in the distribution system. The IDSO would be analogous to ISOs and
RTOs in the transmission system.16
NERC’s reliability standards are meant to ensure an Adequate Level of Reliability (ALR) for the
bulk power system during normal operating conditions and following localized disturbances such
as lightning strikes.17 For economic reasons, some risk of occasional power loss is accepted in
reliability planning. A common goal is to limit outages to no more than 1 day every 10 years
under normal operating conditions.18
Achieving ALR is not the same goal as preventing all brownouts and blackouts. Bulk power
system outages still could occur when reliability standards are fully met. These outages might
follow a major event such as a hurricane, tornado, or wildfire affecting large areas of the bulk
power system.
System: Identifying the Source of Electric Service Interruptions in the US,” IET Generation, Transmission &
Distribution, vol. 13, no. 5 (2019). These findings are consistent with other analyses that the vast majority of power
outages occur in distribution systems.
14 For a discussion of how distribution system outages can be impactful, see CRS Report R42696, Weather-Related
Power Outages and Electric System Resiliency, by Richard J. Campbell.
15 Definitions for microgrids vary, but they generally are understood to include sources of electricity supply and
demand within defined electrical boundaries. Some are capable of operating in isolation of the bulk power system. For
additional information on microgrids, see DOE, “How Microgrids Work,” June 17, 2014.
16 The independent distribution system operator concept is described in Farrokh Rahimi and Sasan Mokhtari, “From
ISO to DSO,” Public Utilities Fortnightly, June 2014, and James Tong and Jon Wellinghoff, “Rooftop Parity: Solar for
Everyone, Including Utilities,” Public Utilities Fortnightly, August 2014.
17 While cybersecurity is an important aspect of power system reliability that NERC addresses, it is outside the scope of
this report. That topic is discussed in CRS Report R43989, Cybersecurity Issues for the Bulk Power System, by Richard
J. Campbell.
18 This target is called loss of load expectation (LOLE) and is calculated using estimates of generation, demand, and the
probabilities of outages on system components. The 1 outage day every 10 years target is equivalently expressed as 0.1
days/year. A technical discussion of LOLE methods in the context of variable sources such as wind and solar is
available in NERC, Methods to Model and Calculate Capacity Contributions of Variable Generation for Resource
Adequacy Planning, March 2011.
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Electricity System Resilience
An emerging topic of interest is the concept of electricity system resilience. This term is somewhat similar to
reliability in that both terms suggest that power will be available when it is wanted. The term resilience, however,
tends to be used in a broader sense and often in the context of disasters. As the National Academies noted
Resilience is not just about lessening the likelihood that these outages will occur. It is also about limiting
the scope and impact of outages when they do occur, restoring power rapidly afterwards, and learning
from these experiences to better deal with events in the future.19
Whereas the electric power industry has developed technical standards and metrics for reliability that are suitable
for regulatory purposes, a technical definition of resilience is still under development. FERC initiated Docket No.
AD18-7-000 on January 8, 2018, proposing to define resilience as “the ability to withstand and reduce the
magnitude and/or duration of disruptive events, which includes the capability to anticipate, absorb, adapt to,
and/or rapidly recover from such an event.”20
In 2021, the U.S. Government Accountability Office recommended certain actions U.S. Department of Energy
(DOE) and FERC could take to promote resilience to climate change impacts on the electricity system.21
This report does not discuss power system resilience in depth, although some system operations and other
activities that support reliability also might support power system resilience. This report uses the terms reliability
and reliability standard as those terms are currently used by NERC.
Generally, factors that increase uncertainty reduce reliability, and factors that reduce uncertainty
increase reliability. Wind and solar are types of variable renewable energy sources of electricity,
and weather is a key source of uncertainty for forecasts of generation from these sources. In
contrast, base load power generation such as coal and nuclear have long-lasting, on-site fuel
supplies that reduce the uncertainty about their availability.22 This difference has raised questions
about how to integrate large amounts of VRE sources into the existing bulk power system, since it
was not originally designed to accommodate large amounts of weather-dependent sources of
electricity.23
Figure 3 shows typical patterns for electricity generation for wind and solar sources in the United
States. Onshore wind generation tends to peak overnight and during winter and early spring
months (i.e., November-April).24 Solar generation, on the other hand, tends to be highest during
the middle of the day and during the late spring and summer (i.e., May-August). Though these
typical patterns are well established for most of the United States, actual generation from wind
and solar sources at any particular moment will depend upon specific weather conditions.
19 National Academies of Sciences, Engineering, and Medicine, Enhancing the Resilience of the Nation’s Electricity
System, The National Academies Press, 2017, p. 1, https://doi.org/10.17226/24836.
20 FERC, Order Terminating Rulemaking Proceeding, Initiating New Proceeding, and Establishing Additional
Procedures, January 8, 2018, Para. 18, https://ferc.gov/CalendarFiles/20180108161614-RM18-1-000.pdf.
21 U.S. Government Accountability Office, Electric Grid Resilience: Climate Change Is Expected to Have FarReaching Effects and DOE and FERC Should Take Action, March 10, 2021.
22 On-site coal stockpiles typically can supply 2-3 months of continuous generation. Nuclear fuel rods typically are
used for up to six years before being removed. Refueling cycles of 18 or 24 months are common, during which a
portion of the reactor’s fuel rods are replaced. DOE, Staff Report to the Secretary on Electricity Markets and
Reliability, 2017, https://www.energy.gov/downloads/download-staff-report-secretary-electricity-markets-andreliability.
23 Many discussions of the challenges surrounding use of VRE exist. See, for example, DOE, Staff Report to the
Secretary on Electricity Markets and Reliability, 2017, https://www.energy.gov/downloads/download-staff-reportsecretary-electricity-markets-and-reliability.
24 Hawaii and parts of California, Oregon, and Washington tend to have maximum generation from wind sources
during summer months due to unique meteorological circumstances. Pacific Northwest Laboratory, Wind Energy
Resource Atlas of the United States, 1986.
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Figure 3. Generation Patterns for U.S. Wind and Solar Sources
Sources: CRS. Daily generation profiles based on California Independent System Operator, Renewables Trend,
May 13, 2019, http://www.caiso.com/TodaysOutlook/Pages/supply.aspx. Daily peak demand based on national
data from EIA, “U.S. Electric System Operating Data,” https://www.eia.gov/realtime_grid/#/summary/demand.
Seasonal profiles and peak demand based on 2018 data for total United States, EIA, “Electricity Data Browser,”
https://www.eia.gov/electricity/data/browser/.
Notes: The y-axis represents the potential energy generation from each individual source, from a minimum of
zero to a maximum defined by the installed capacity of the source. Actual generation from any individual source
will depend on local weather conditions and other factors. Wind seasonal generation patterns vary by region, so
the pattern shown here may not apply in all cases. The daily pattern for solar is typical of south-facing
photovoltaic panels in the United States. West-facing panels tend to reach maximum generation 1-2 hours later
in the day.
Changing Electricity Generation Profile
The electric power sector is increasing its use of sources associated with more uncertainty in
availability. According to EIA, combined generation from wind and utility-scale solar sources
increased from 1% of total electricity generation in 2008 (the first year in which this share was
greater than 1%) to 13% of total electricity generation in 2021.25 Of the generation in 2021 from
wind and utility-scale solar sources, 77% came from wind. Conventional sources such as coal,
natural gas, and nuclear comprised a large majority of power generation over this time period.
The annual share of generation from different sources from 2008 to 2021 in shown in Figure 4.
25 CRS analysis of data from EIA, “Electricity Data Browser,” accessed May 19, 2022, https://www.eia.gov/electricity/
data/browser/. EIA began collecting data on generation from small-scale solar PV sources in 2014. The statistic
reported for 2021 includes both large-scale and small-scale solar sources. Typically, small-scale solar PV sources are
connected to the distribution system, not the bulk power system.
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Figure 4. Annual Net U.S. Electricity Generation by Source, 2008-2021
Percent of total
Source: CRS analysis of data from EIA, “Electricity Data Browser,” accessed May 19, 2022, https://www.eia.gov/
electricity/data/browser/.
Notes: Some EIA categories are combined. Other = petroleum liquids, petroleum coke, other gas, and other.
Other Renewables = wood and wood-derived fuels, landfill gas, biogenic municipal solid waste, other waste
biomass, geothermal, conventional hydroelectric, and hydroelectric pumped. “Solar” includes generation from
small-scale solar beginning in 2014.
Some regions of the country have much higher levels of wind and solar generation than the nation
as a whole. Similarly, some times of year have much higher levels of wind and solar electricity
generation than the annual averages. For example, during brief periods in some regions, wind and
solar sources have provided a large majority of the energy for electricity generation. Some
examples are
On April 30, 2022, generation from renewable energy sources equaled 100% of
electricity demand in the California Independent System Operator, the RTO
covering most of California, for about 15 minutes. About two-thirds of the
generation from renewable energy sources was from solar.26
On March 29, 2022, generation from wind energy sources supplied 89% of
electricity demand in the Southwest Power Pool (SPP), the RTO covering many
central states.27
On April 10, 2022, generation from wind energy sources supplied 69% of
electricity demand in the Electric Reliability Council of Texas, the RTO covering
most of Texas.28
26 Ryan Kennedy, “For the First Time in History, California’s Demand Was 100% Matched by Renewable Energy
Generation,” PV Magazine, May 2, 2022.
27 Southwest Power Pool (SPP) press release, “SPP Sets Regional Records for Renewable Energy Production,” March
29, 2022.
28 Electric Reliability Council of Texas, “Fact Sheet,” April 2022.
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These events all set regional records for maximum share of generation from renewable sources,
and the bulk power system maintained reliability during them. Although some may contend these
events demonstrate how high levels of wind and solar can be used without affecting reliability,
extrapolating these events to national, annual levels is complicated by several factors. First, these
events were all short lived, typically fifteen minutes or less. Further, these events all occurred
when electricity demand was relatively low, during cool months and (except for SPP) on
weekends. During times of the year when electricity demand is high, such as the summer cooling
season, the share of electricity generation from renewable sources is lower.29 For example, SPP
has reported that during its peak demand hours in 2016, wind supplied 11% of generation while
conventional sources such as coal (47%) and natural gas (33%) supplied the majority of
electricity.30 The seasonality of VRE availability also likely contributed to these record-setting
events, especially for wind, which tends to have maximum generation during winter and spring
months.
Balancing Variable Renewable Energy
Electricity is generated essentially as a just-in-time commodity, due to limited energy storage
capacities. If electricity supply and demand differ by too much, system components and customer
equipment could be damaged, leading to system instability or potential failure. The operations
that keep electricity supply and demand within acceptable levels are known as system load
balancing.
Balancing involves increasing or decreasing output from generators according to system
conditions over timescales of minutes to hours, and it is a critical aspect of maintaining reliability.
Balancing authorities, discussed in the Appendix, issue orders to generators to change their
output as needed to maintain reliability. Balancing authorities can be utilities, or RTOs can act as
balancing authorities in the regions where they exist. The rules for selecting which generators
must increase or decrease output typically reflect an approach known as security-constrained
economic dispatch (SCED). Under SCED, system operators ensure that electricity is produced at
the lowest overall cost while respecting any transmission or operational constraints.31 When
generation from a low-cost source would jeopardize reliability, a higher-cost source is used. In
other words, SCED has two goals: affordability and reliability.
SCED favors sources with low operating costs, and wind and solar sources do not have to pay for
fuel. As a result, wind and solar sources typically generate the maximum amount of electricity
they can at any moment.32 Balancing typically involves quickly increasing or decreasing output
from other sources in response to variable output from wind and solar sources. The capability to
29 VRE sources generally are used to the maximum extent possible, for economic reasons, as described in the section
“Balancing Variable Renewable Energy.” Unlike other types of sources, the potential supply of electricity from VRE
sources usually does not change in response to total electricity demand. Thus, when demand increases and more
electricity in total is generated to meet that demand, the relative share of generation from VRE sources decreases. If
total demand for electricity at any moment is 10,000 MW and 5,000 MW of electricity from VRE sources is available,
the share of generation from VRE sources would likely be 50%. If, however, total demand were 50,000 MW, the share
of generation from VRE sources would likely be 10%.
30 SPP Supply Adequacy Working Group, Wind and Solar Report, May 23, 2017, https://www.spp.org/documents/
53721/sawg%20approved_wind%20and%20solar%20report.pdf.
31 In EPACT05, Congress required FERC to study SCED and report its findings to Congress. That report is FERC,
Security Constrained Economic Dispatch: Definition, Practices, Issues and Recommendations, July 31, 2006.
32 Exceptions do exist. Many such instances, known as curtailment, have been associated with limited transmission
capacity.
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quickly change output is known as ramping, and electricity sources differ in their ramping
capability.
System operators use a variety of electricity sources to balance generation from wind and solar
sources. Some may be more commonly used in certain regions of the country, depending on local
factors. Each has different benefits and limitations, some of which are summarized below.
Reciprocating internal combustion engines have seen an increase in installed
capacity since 2000, partly in response to higher levels of generation from wind
and solar sources.33 These sources have high ramping capabilities and use mature
technologies. They usually use natural gas or fuel oil as fuel, so they have
associated fuel costs and environmental impacts.34
Steam turbines, usually fueled by coal or nuclear energy, historically have been
operated at steady, high output levels, barring maintenance needs, because that is
the most efficient and lowest cost operational mode for them. These sources are
capable of ramping to some extent. This operational mode may provide revenue
for certain sources located in regions of the country with low wholesale
electricity prices. It also might result in higher costs for electricity from these
sources, compared to when they are not ramped.35
Hydropower can be ramped up or down relatively quickly, has relatively few
associated greenhouse gas emissions, and is relatively inexpensive to operate.
Existing hydropower resources are concentrated in some regions of the country,
and development of new resources may face barriers such as concerns over the
environmental impact of dams. Proposals to expand U.S. hydropower capacity
have focused on increasing the electricity generating capacity at existing
hydropower dams and adding electricity generating capacity to existing nonpowered dams.36 Drought and other weather factors can affect the availability of
hydropower, though not over the same short timescales over which weather
affects wind and solar availability.
Wind and solar sources located in one area can potentially balance wind and solar
sources in other areas, since it is rare to have cloudy skies or calm winds over
broad regions of the country almost simultaneously. This could have the benefit
of using sources with zero fuel costs and zero emissions for balancing; however,
33 Suparna Ray, Natural Gas-Fired Reciprocating Engines Are Being Deployed More to Balance Renewables, EIA,
February 19, 2019, https://www.eia.gov/todayinenergy/detail.php?id=37972. Reciprocating Internal Combustion
Engines work through pistons, similar to internal combustion engines used in vehicles. In contrast, most base load
plants using natural gas rely, at least in part, on steam-driven turbines.
34 Some other benefits and limitations are discussed in Chris Warren, “20 Power Companies Examine the Role of
Reciprocating Internal Combustion Engines for the Grid,” EPRI Journal, September 26, 2018, http://eprijournal.com/
start-your-engines/.
35 Power plant operators do not often publicly discuss operational decisions because they can contain proprietary
information. Aggregated and anonymous industry reports document how flexible operations like ramping are
increasing. For example, see Peter Maloney, “How Market Forces Are Pushing Utilities to Operate Nuclear Plants
More Flexibly,” Utility Dive, October 4, 2016, https://www.utilitydive.com/news/how-market-forces-are-pushingutilities-to-operate-nuclear-plants-more-flex/427496/.
36 For a discussion of hydropower and proposals to expand U.S. hydropower capacity, see, for example, CRS Report
R42579, Hydropower: Federal and Nonfederal Investment, by Kelsi Bracmort, Adam Vann, and Charles V. Stern, and
DOE, Hydropower Vision Report, July 2016.
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existing electricity transmission system constraints limit the extent to which this
is possible.37
Energy storage can be used for balancing because it stores electricity during
periods of high supply and then provides electricity when supply is low. Many
experts also see storage as a way to address the daily variability shown in Figure
3 and thereby expand the utilization of installed wind and solar sources.38 Many
energy storage types are relatively expensive and not currently deployed in large
amounts, although energy storage deployment (especially battery storage) is
growing.39 Energy storage can be co-located with wind or solar generators, or it
can be located at other sites in the power system or the distribution system.40
Demand response, sometimes called demand-side management, involves
adjusting electricity demand in response to available supply. This is counter to
how the power system historically has been operated, but has become more
commonly used.41 Demand response includes programs in which electricity
consumers voluntarily reduce their usage in exchange for financial compensation.
Demand response can be a low-cost balancing option because it does not require
electricity generation; however, it comes at a social cost because consumers do
not use electricity at their preferred time.
The electric power sector is working to improve the use of weather and power
forecasting in system balancing. For example, MISO changed its wholesale
electricity market rules in 2011 to create a Dispatchable Intermittent Resources
program.42 This program allows wind and solar sources to make use of their own
generation forecasts and offer generation at five-minute intervals. Previously,
offers had to be made on an hourly basis. This was creating inefficiencies in
using VRE since their output can vary over the course of an hour. Improved
37 Existing boundaries between balancing authorities also can pose hurdles to using remote wind and solar sources for
balancing. Some Western states have created an Energy Imbalance Market to address such market hurdles. See Aaron
Larson, “How Does the Western Energy Imbalance Market Work?,” https://www.powermag.com/how-does-thewestern-energy-imbalance-market-work/.
38 For example, “grid-scale storage helps with renewable integration, allowing higher renewable energy levels than
would otherwise be possible.” Ben Fowke, Chairman, President, and Chief Executive Officer of Xcel Energy in written
testimony, U.S. Congress, Senate Committee on Energy and Natural Resources, Hearing on Expanded Deployment of
Grid-Scale Energy Storage, 116th Cong., June 4, 2019.
39 Current policy discussion is focused on how energy storage might balance daily variability of wind and solar sources,
but some energy storage types potentially could balance seasonal variability as well. For a discussion of energy storage,
see CRS Report R42455, Energy Storage for Power Grids and Electric Transportation: A Technology Assessment, by
Paul W. Parfomak, and CRS Report R45980, Electricity Storage: Applications, Issues, and Technologies, by Richard J.
Campbell.
40 In Order No. 841, issued February 15, 2018, FERC defines an energy storage source as “a resource capable of
receiving electric energy from the grid and storing it for later injection of electric energy back to the grid.” The
definition does not specify location, and FERC further clarifies that “electric storage resources located on the interstate
transmission system, on a distribution system, or behind the meter fall under this definition.” FERC, Order No. 841:
Electric Storage Participation in Markets Operated by Regional Transmission Organizations and Independent System
Operators, February 15, 2018, paragraph 29.
41 Demand response in energy markets is discussed in CRS Report R43093, Electricity Markets—Recent Issues in
Market Structure and Energy Trading, by Richard J. Campbell.
42 Jasmin Melvin, “FERC OKs MISO Plan to Extend ‘Dispatchability’ Requirement to Solar Resources,” S&P Global
Market Intelligence, June 11, 2020.
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forecasting could result in increased use of low-cost wind and solar sources, but
forecasting methodologies are still being optimized for this purpose.
The above considerations apply to bulk power system balancing today. Technological or policy
developments could alter how system balancing is conducted in the future. Additionally, if wind
and solar sources provided even larger shares of overall generation, new benefits or limitations
for each balancing source type could emerge.
Federal Government Activities Affecting Reliability
and Balancing
Work at the federal level to address reliability needs associated with increased use of wind and
solar sources has been underway for some time. For example, NERC created a task force in
December 2007 to study the integration of VRE and identify gaps in reliability standards.43
The federal government undertakes actions in addition to the development and enforcement of
reliability standards that affect electric reliability. FERC regulates interstate electricity
transmission, which can be a key determinant of what sources are available to balance wind and
solar. FERC also regulates wholesale electricity transactions, and competitive markets in most
regions of the country. Market rules, including how SCED is implemented, can influence which
individual generators are used for system balancing. Market prices can directly affect project
revenues and influence investment decisions. Additionally, Congress funds projects and programs
that support technology development and deployment, including for sources and operations that
improve reliability.
Some examples demonstrate the breadth of federal activities indirectly related to reliability.
EPACT05 created Section 219 of the Federal Power Act, directing FERC to
establish financial incentives for certain electricity transmission investments.
FERC’s resulting rule became effective in 2006 and includes provisions allowing
higher rates of return, accelerated depreciation, and full cost recovery, all for
investments and activities that FERC approves on a case-by-case basis.44
Transmission investment has increased since the passage of EPACT05, although
there may be many factors driving this investment.45 On March 21, 2019, FERC
opened an inquiry on potential changes to its transmission incentive policy. The
agency continues to move forward with proposed reforms.46
In 2011, FERC issued a rule, Order No. 1000, revising requirements related to
new transmission projects.47 Among other revisions, Order No. 1000 increased
the weight given to achieving public policy requirements when FERC considers
43 NERC, Accommodating High Levels of Variable Generation, April 2009.
44 FERC, Order No. 679: Promoting Transmission Investment Through Pricing Reform, July 20, 2006,
http://elibrary.ferc.gov/idmws/common/opennat.asp?fileID=11089376.
45 Lori Aniti, Utilities Continue to Increase Spending on Transmission Infrastructure, EIA, February 9, 2018,
https://www.eia.gov/todayinenergy/detail.php?id=34892.
46 FERC, “News Release: FERC Opens Inquiry on Improvements to Electric Transmission Incentives Policy,” press
release, March 21, 2019, and FERC, “FERC Proposes Further Reforms to Electric Transmission Incentives Policy,”
April 15, 2021.
47 FERC, “Order No. 1000—Transmission Planning and Cost Allocation,” updated November 9, 2021.
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approval of transmission projects.48 An example of a public policy requirement
might be a state requirement that a specified share of electricity sales come from
renewable sources, a policy commonly known as a renewable portfolio
standard.49 In July 2021, FERC began a new process for additional reform to its
transmission policies.50
Several FERC orders demonstrate how market rules are changing in response to
increased need for balancing and ramping. Order No. 745 allows demand
response to earn compensation from wholesale electricity markets for providing
energy services to balance the power system in day-ahead and real-time
markets.51 Order No. 841 allows energy storage systems to earn compensation
from wholesale electricity markets for providing any energy, capacity, and
essential reliability services they are capable of providing.52 Implementation of
Order No. 841 might lead to greater deployment of energy storage which could
improve balancing.
Various grant programs administered by DOE have supported the development of
new technologies that can balance wind and solar sources or support reliability in
other ways. These include research and development into electricity generators;
wind forecast models and methodology; power electronics for solar sources; and
standards for interconnection into the bulk power system.53
DOE’s Office of Energy Efficiency and Renewable Energy has funded research
meant to improve short-term weather forecasting specifically related to wind
power forecasts in two Wind Forecast Improvement Projects. DOE reports that
advances made during this research include improved observations of
meteorological data and improved methodologies for using those data in wind
forecasts.54
48 For a discussion, see CRS Report R41193, Electricity Transmission Cost Allocation, by Richard J. Campbell and
Adam Vann.
49 Thirty states and the District of Columbia are implementing mandatory renewable portfolio standards, and an
additional eight states have voluntary versions. Texas and Iowa have portfolio standards that set a requirement in terms
of installed capacity, not share of electricity sales. DSIRE, Renewable & Clean Energy Standards, September 2020,
https://ncsolarcen-prod.s3.amazonaws.com/wp-content/uploads/2020/09/RPS-CES-Sept2020.pdf. For discussion of this
policy, see CRS Report R45913, Electricity Portfolio Standards: Background, Design Elements, and Policy
Considerations, by Ashley J. Lawson.
50 FERC, “News Release: FERC Begins Reform Process to Build the Transmission System of the Future,” press
release, July 15, 2021, and FERC, “FERC Issues Transmission NOPR Addressing Planning, Cost Allocation,” press
release, April 21, 2022.
51 FERC, Order No. 745: Demand Response Compensation in Organized Wholesale Energy Markets, March 15, 2011,
https://www.ferc.gov/sites/default/files/2020-06/Order-745.pdf.
52 FERC, Order No. 841: Electric Storage Participation in Markets Operated by Regional Transmission Organizations
and Independent System Operators, February 15, 2018.
53 The authorization and appropriations history for some of these programs is summarized in CRS Report R40913,
Renewable Energy and Energy Efficiency Incentives: A Summary of Federal Programs, by Lynn J. Cunningham. Many
of these efforts were organized within DOE as part of its Grid Modernization Initiative. Information on that effort is
available at https://www.energy.gov/grid-modernization-initiative.
54 DOE, WFIP NOAA Final Report, April 30, 2014; and NOAA, Wind Forecast Improvement Project 2 (WFIP 2) in
Complex Flow.
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The Infrastructure Investment and Jobs Act (P.L. 117-58)
On November 15, 2021, President Biden signed the Infrastructure Investment and Jobs Act (IIJA;
P.L. 117-58), sometimes referred to as the Bipartisan Infrastructure Law. This broad infrastructure
law addressed a range of issues, including surface transportation, transit, water infrastructure,
broadband, and minority business. IIJA also addressed a number of energy issues, including
issues related to electricity reliability.55 As of July 2022, most of these IIJA provisions are in early
stages of implementation, and their eventual impact on electric reliability in the United States is
unknown. One focus area in IIJA is electricity transmission. New authorities were given to DOE
and FERC with the goals of addressing electric reliability, resilience, and promotion of renewable
energy.56
Section 40105 of IIJA revises the process for designation of a National Interest Electric
Transmission Corridor (NIETC) by DOE.57 A key revision allows for an NIETC designation that
may lead to new interstate transmission lines connecting regions with strong wind or solar
resources. Another key change in the section enhances FERC’s “backstop” siting authority for
transmission lines in NIETCs. This would allow FERC to supersede traditional state permitting of
transmission facilities and issue a permit for the construction and operation of certain interstate
facilities under defined circumstances, including when a state has denied an applicant’s request to
site transmission facilities. It is unclear when or if FERC may use this authorization.58
Section 40106 establishes the “Transmission Facilitation Program,” under which DOE can
facilitate the construction of electric power transmission lines and related facilities.59 Under this
program, DOE may enter into a capacity contract (for no more than 40 years or 50% of the total
capacity) with respect to an eligible transmission project; issue a loan to an eligible entity for an
eligible transmission project; or participate with an eligible entity in designing, developing,
constructing, operating, maintaining, or owning an eligible transmission project. Thus, under a
capacity project, DOE could be closely involved in operational support of eligible transmissionline construction. Such an arrangement potentially could help move a transmission project from
proposal to construction, as a transmission project is unlikely to be built without significant
customer commitment to its use.
IIJA also provided appropriations for research, development, and deployment of various
technologies that could potentially support electric reliability. Examples include:
$8 billion for Regional Clean Hydrogen Hubs which IIJA established (hydrogen
can potentially be used as energy storage and for balancing);60
55 For an overview of energy issues in the Infrastructure Investment and Jobs Act (IIJA), see CRS Report R47034,
Energy and Minerals Provisions in the Infrastructure Investment and Jobs Act (P.L. 117-58), coordinated by Brent D.
Yacobucci.
56 For additional information on IIJA transmission provisions, see CRS Insight IN11821, IIJA: Efforts to Address
Electric Transmission for Reliability, Resilience, and Renewables, by Richard J. Campbell.
57 For background information on National Interest Electric Transmission Corridors, see CRS In Focus IF11411, The
Legal Framework of the Federal Power Act, by Adam Vann, and DOE, “DOE Issues Two Draft National Interest
Electric Transmission Corridor Designations,” April 26, 2007.
58 Catherine Morehouse, “FERC Will Hold Off on Flexing Transmission Authority, Clements Says,” Politico, January
25, 2022.
59 DOE, “Transmission Facilitation Program,” May 10, 2022.
60 42 U.S.C. §16161a.
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$5 billion for the Program Upgrading Our Electric Grid and Ensuring Reliability
and Resiliency which IIJA established;61
$505 million for several energy storage demonstration projects;62
$125 million for incentives for qualified hydropower;63
$10 million for a pumped storage hydropower demonstration project that is “able
to store electricity generated by intermittent renewable electricity projects located
on Tribal land.”64
Potential Issues for Congress
Congress has held hearings related to the changes in the electricity generation profile of the
country, and some Members raised concerns about reliability during these hearings.65 Members
may continue to examine reliability issues moving forward, in light of projections that wind and
solar will become an increasingly larger share of electricity generation. For example, EIA’s
projection of existing law and regulations shows wind and solar sources contributing 36% of
electricity generation in 2050.66 Potential power outages—such as those that affected parts of
Texas and other states in February 2021—might prompt additional congressional oversight
activity.67
Preparing for higher levels of generation from wind and solar might require modified approaches
to maintaining electric reliability, given the variable nature of these sources. The existing
regulatory framework can accommodate some changes since FERC and NERC have authority to
initiate development of new reliability standards. For example, NERC has raised the issue of
whether it should develop new reliability metrics in light of the increasing use of VRE for
electricity generation.
In addition to its capacity supply assessment, NERC’s Reliability Assessment
Subcommittee should lead the electric industry in developing a common approach and
identify metrics to assess energy adequacy. As identified in this assessment, the changing
resource mix can alter the energy and availability characteristics of the generation fleet.
61 42 U.S.C. §18712(b).
62 The funded energy storage demonstrations are authorized at 42 U.S.C. §17232(c) and 42 U.S.C. §17232(d).
63 42 U.S.C. §15881.
64 42 U.S.C. §17232(e)(2)(iii).
65 See, for example, U.S. Congress, Senate Committee on Energy and Natural Resources, Keeping the Lights On—Are
We Doing Enough to Ensure the Reliability and Security of the U.S. Electric Grid?, 113th Cong., April 10, 2014, S.
Hrg. 113-271; U.S. Congress, House Committee on Energy and Commerce, Subcommittee on Energy and Power,
Hearing on Discussion Draft Addressing Energy Reliability and Security, 114th Cong., May 19, 2015; U.S. Congress,
House Committee on Energy and Commerce, Subcommittee on Energy, Powering America: Defining Reliability in a
Transforming Electricity Industry, 115th Cong., September 14, 2017; U.S. Congress, Senate Committee on Energy and
Natural Resources, Hearing to Examine the Electricity Sector in a Changing Climate, 116th Cong., March 5, 2019; and
U.S. Congress, House Committee on Energy and Commerce, Subcommittee on Oversight and Investigations, Power
Struggle: Examining the 2021 Texas Grid Failure, 117th Cong., 1st sess., March 24, 2021.
66 EIA, Annual Energy Outlook 2022, March 3, 2022, p. 19, https://www.eia.gov/outlooks/aeo/pdf/
AEO2022_Narrative.pdf. Factors that could increase the use of wind and solar sources above this level might be
missing from EIA’s analysis. Such factors include new state or federal policies that might be enacted and unexpected
cost declines for wind and solar sources or energy storage technologies.
67 For a discussion of the causes of the February 2021 power outages, see FERC, NERC, and Regional Entity Staff
Report, The February 2021 Cold Weather Outages in Texas and the South Central United States, November 2021.
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Additional analysis is needed to determine energy sufficiency, particularly during off-peak
periods and where energy-limited resources are most prominent. 68
Congress could choose to provide guidance for FERC and NERC activities in this area.
Additionally, Congress could assess whether the existing regulatory framework is sufficient to
maintain reliability given the changing mix of energy sources used for electricity generation. An
emerging area of interest is the interdependency of electric systems and natural gas systems.69
Increased use of natural gas for electricity generation raises potential reliability risks that are
distinct from those associated with wind and solar. However, the two may be linked through the
widespread use of natural gas to balance wind and solar.
Congress continues to debate the desired amount of wind and solar energy used for electricity
generation. For example, H.R. 5376, the Build Back Better Act, passed by the House in
November 2021, and the Inflation Reduction Act of 2022, released by Senate Majority Leader
Schumer and Senator Manchin on July 27, 2022, would provide several tax credits aimed at
increasing the use of renewable energy sources such as wind and solar.70 Congress could consider
how financial incentives for renewable energy could potentially take electric reliability into
account.
Agency actions affecting the power sector may be of interest as well. For example, the U.S.
Environmental Protection Agency (EPA) may regulate greenhouse gas emissions in the power
sector under the Clean Air Act. Such regulations potentially could lead to more favorable market
conditions for wind and solar energy sources relative to fossil fuels. Any potential market impact
of EPA regulations on power sector emissions will be influenced by the specifics of how they
may be implemented. The scope of regulatory tools available to EPA, and any regulations
themselves, could be influenced by the U.S. Supreme Court’s ruling in West Virginia v. EPA.71
Congress could consider clarifying the scope of EPA’s authority to regulate greenhouse gas
emissions from the power sector, including how (if at all) EPA alone or together with other
agencies might address any potential reliability concerns arising from an agency action.72
As noted above, most power outages occur on local electricity distribution systems, and these are
generally regulated by state or local governments. Congress could consider examining whether
distribution system reliability warrants federal action. This might involve studies of the factors
(e.g., weather, aging infrastructure, VRE) that result in power outages or promotion of best
practices for distribution system reliability. It also might include federal financial support for
68 NERC, 2018 Long-Term Reliability Assessment, December 20, 2018, p. 9, https://www.nerc.com/pa/RAPA/ra/
Reliability%20Assessments%20DL/NERC_LTRA_2018_12202018.pdf.
69 See, for example, NERC, Special Reliability Assessment: Potential Bulk Power System Impacts Due to Severe
Disruption on the Natural Gas System, November 2017; and Robert Walton, “North American Energy Standards Board
to Tackle Gas-Electric Coordination in 2022,” Utility Dive, December 15, 2021. Additionally, NERC and FERC found
forced shutdowns of power plants and natural gas supply were “inextricably linked” during the February 2021 power
outages in Texas and other states. See FERC, NERC, and Regional Entity Staff Report, The February 2021 Cold
Weather Outages in Texas and the South Central United States, November 2021, p. 172.
70 For a discussion of energy tax credits included in the House-passed Build Back Better Act, see CRS Report R46960,
Tax Provisions in the Build Back Better Act: Rules Committee Print 117-18, coordinated by Molly F. Sherlock. For a
discussion of energy tax credits included in the Inflation Reduction Act of 2022, see CRS Report R47202, Tax
Provisions in the Inflation Reduction Act of 2022 (H.R. 5376), coordinated by Molly F. Sherlock.
71 For an overview of West Virginia v EPA, see CRS Legal Sidebar LSB10791, Supreme Court Addresses Major
Questions Doctrine and EPA’s Regulation of Greenhouse Gas Emissions, by Kate R. Bowers.
72 Some Members have raised concerns about potential reliability issues arising from environmental regulations. See,
for example, House Energy and Commerce Committee Republicans, “House Republicans Demand Answers from EPA
on Life Threatening Blackouts and Grid Instability,” press release, July 12, 2022.
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projects or practices that improve reliability of distribution systems or encouraging new
operational regimes such as independent distribution system operators. (See earlier discussion of
this issue in text box, “Distribution System Reliability.”)
Congress also might consider acting on the emerging and related issue of electric resilience.
Some support for an enhanced federal role in electricity system resilience exists. For example, the
National Academies recommend
Congress and the Department of Energy leadership should sustain and expand the
substantive areas of research, development, and demonstration that are now being
undertaken by the Department of Energy’s Office of Electricity Delivery and Energy
Reliability and Office of Energy Efficiency and Renewable Energy, with respect to grid
modernization and systems integration, with the explicit intention of improving the
resilience of the U.S. power grid.73
Electric resilience faces challenges, such as higher costs and technical issues challenges (e.g.,
developing appropriate scenarios with sufficient data granularity around which to plan for
resilience).
73 National Academies of Sciences, Engineering, and Medicine, Enhancing the Resilience of the Nation’s Electricity
System, 2017, p. 4, https://doi.org/10.17226/24836.
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Appendix. Key Reliability Concepts for
Policymakers
Electric reliability encompasses short-term and long-term aspects as shown in Figure A-1.
System operators and reliability planners, governed by reliability standards from the North
American Electric Reliability Corporation (NERC), have different practices in place to address
reliability over these various timescales.
Figure A-1. Power System Reliability Timescales
Source: DOE, Staff Report to the Secretary on Electricity Markets and Reliability, August 2017, p. 62,
https://www.energy.gov/downloads/download-staff-report-secretary-electricity-markets-and-reliability.
Notes: AC = alternating current; T&D = transmission and distribution systems.
Reliability over Different Timescales
At the smallest timescales, typically seconds or less, reliability requires factors such as frequency
control, voltage support, and ramping capability.74 These are often automatic responses of power
system components. NERC refers to these factors as Essential Reliability Services (ERS), and
they are sometimes called ancillary services. Historically, many ERS were provided as a natural
consequence of the physical operational characteristics of steam turbines. Wind and solar
generators do not inherently provide ERS in the same way. They require additional electrical
components to do so, and these are being more commonly deployed. In some cases, FERC has
mandated the use of technologies that allow wind and solar to provide ERS.75
Balancing, described in the main body of this report, typically occurs over minutes to hours.
Unlike ERS, balancing typically requires action by a system operator.
Long-term aspects of reliability relate to planning for energy and transmission needs over months
to years. This is sometimes referred to as resource adequacy. Policy goals, such as preferences for
certain electricity sources over others, tend to influence long-term reliability planning more than
shorter-term reliability aspects.
74 For more background, see FERC, Reliability Primer, https://www.ferc.gov/media/2135.
75 For a discussion of the technical and market issues around ERS, see Department of Energy, Staff Report to the
Secretary on Electricity Markets and Reliability, 2017, https://www.energy.gov/downloads/download-staff-reportsecretary-electricity-markets-and-reliability.
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Planning for resource adequacy involves forecasts of electricity supply and demand. For variable
renewable energy (VRE) like wind and solar sources, these forecasts require assumptions about
wind and solar availability. Reliability planners commonly use planning reserve margins to assess
whether planned generation and transmission capacity will be sufficient to supply electricity
demand. A planning reserve margin is the difference between expected peak demand and
available generating capacity at the peak period in each forecast year, normalized to peak demand
and expressed as a percentage.76 NERC typically recommends reserve margins between 10% and
20%.77 Reserve margins are calculated months or years ahead as part of assessments of whether
and where reliability concerns might exist. High planning reserve margins may indicate a
likelihood that reliability will be maintained, but, especially when variable sources are present,
they might not be predictive. That is, a high planning reserve margin does not guarantee
reliability and a low planning reserve margin does not guarantee power disruptions.
At the national level, NERC annually assesses resource adequacy over a 10-year forecasting
window. NERC uses historic VRE generation data in its assessment and has noted “methods for
determining the on-peak availability of wind and solar are improving with growing performance
data.”78 NERC also uses probabilistic evaluations that can account for the uncertainty in
availability for VRE and other resources (e.g., natural gas). In its 2020 Long-Term Reliability
Assessment, NERC noted
The traditional methods of assessing resource adequacy at peak load times may not
accurately or fully reflect the ability of the new resource mix to supply energy and reserves
for all hours. Energy limitations can exist, requiring probabilistic analysis methods to
identify risks to reliability resulting from shortfalls in the conversion of capacity to energy
(energy adequacy). The new resource mix includes natural-gas-fired generation;
unprecedented proportions of nonsynchronous resources, including renewables and battery
storage; [demand response]; smart- and micro-grids; and other emerging technologies.
Collectively, the new resources are more susceptible to energy sufficiency uncertainty.79
Solar eclipses, though rare events, provide opportunities to test the ability of grid operators to
reliably operate the grid when solar sources are unavailable. The August 21, 2017, solar eclipse
that affected many parts of the United States was one such opportunity. According to NERC, no
reliability issues developed during the event, in part because of the measures implemented in
advance by the electric industry.80
Electric Reliability Regulatory Framework
Current electric reliability planning is a coordinated process involving multiple entities and
spanning multiple jurisdictions. These reliability planning organizations share responsibility for,
among other responsibilities, ensuring electricity from wind and solar sources are reliably
integrated into the power system. Table A-1 summarizes these entities and their responsibilities.
76 NERC, M-1 Reserve Margin, accessed August 4, 2022, https://www.nerc.com/pa/RAPA/ri/Pages/
PlanningReserveMargin.aspx.
77 Ibid.
78 NERC, 2017 Long-Term Reliability Assessment, March 1, 2018, p. 5, https://www.nerc.com/pa/RAPA/ra/
Reliability%20Assessments%20DL/NERC_LTRA_12132017_Final.pdf.
79 NERC, 2020 Long-Term Reliability Assessment, December 2020, p. 18, https://www.nerc.com/pa/RAPA/ra/
Reliability%20Assessments%20DL/NERC_LTRA_2020.pdf.
80 NERC, State of Reliability 2018, June 2018, p. 38, https://www.nerc.com/pa/RAPA/PA/
Performance%20Analysis%20DL/NERC_2018_SOR_06202018_Final.pdf.
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Maintaining Electric Reliability with Wind and Solar Sources
Table A-1. Selected Entities Involved in Power System Reliability
Entity
Name
Selected
Reliability
Activities
Enforcement
Mechanism
Oversight
Mechanism
Notes
Federal
Energy
Regulatory
Commission
(FERC)
Approves new or
modified reliability
standards
proposed by
NERC and
conducts
enforcement
activities.
FERC can
enforce
compliance
plans, reliability
enhancements,
and impose civil
penalties for
violations of
reliability
standards.
Commissioners
are nominated
by the President
and confirmed
by the Senate.
Pursuant to EPACT05, FERC may
approve reliability standards that
are developed by the Electric
Reliability Organization (ERO), or
remand standards back for
reconsideration if deemed
inadequate.
FERC rules for investigating
potential violations of reliability
standards are codified in 18 C.F.R.
§1b.
North
American
Electric
Reliability
Corporation
(NERC)
Recommends
reliability
standards for
industry and
annually assesses
long-term (10year) resource
adequacy.
NERC files
Notices of
Penalty with
FERC.
Independent
board oversees
NERC. FERC
may direct
NERC to revise
reliability
standards.
FERC certified NERC as the ERO in
2006. EPACT05 specifies duties of
the ERO regarding reliability
standards.
Regional
Entity
Monitors and
enforces
compliance with
NERC reliability
standards.
Can recommend
penalties on
entities that
violate reliability
standards and/or
fail to mitigate
any violations.
NERC can hear
appeals in case
of disputed
violations.
EPACT05 allows the ERO to
delegate some responsibilities to
Regional Entities. Older documents
may refer to eight regional entities,
but in 2018 the Southwest Power
Pool Regional Entity ceased
operations and its members
transferred into other regional
entities, leaving seven regional
entities.
Balancing
Authority
(BAs)
Maintains real time
power system
reliability in
response to
system conditions.
Enforcement
conducted by
appropriate
Regional Entity.
Mostly
conducted at
the local or
regional level.
BAs as separate entities are not
required by law. Instead, NERC
reliability standards require BAs to
ensure that electricity supply and
demand are balanced in real time.
Entities voluntarily register to take
on these duties. All of the Regional
Transmission Organizations (RTOs)
act as BAs.
Public Utilities
Commissions
(PUCs)
In some states
PUCs approve
power system
investments,
including those
related to power
system reliability.
PUCs have
exclusive
jurisdiction over
distribution
system reliability.
Could include
imposing
penalties or
denying cost
recovery for
damages
associated with
poor reliability.
Some state
utilities
commissioners
are appointed
by governors;
others are
elected.
Some state policies, including those
supporting wind and solar energy
development, might have
consequences for reliability. Some
states are considering adoption of
performance-based rates to provide
financial incentives to utilities with
positive reliability performance.
Source: CRS analysis.
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Maintaining Electric Reliability with Wind and Solar Sources
Notes: Other entities, including generator operators, transmission operators, and reliability coordinators are
involved in reliability planning, but their activities are not shown here. Each of those entities has a different scope
of function in maintaining power system reliability. A full list of the roles of power system entities in maintaining
reliability is available in the NERC Reliability Functional Model at https://www.nerc.com/pa/Stand/pages/
functionalmodel.aspx.
In the Energy Policy Act of 2005 (EPACT05; P.L. 109-58), Congress gave FERC responsibility
for reliability of the grid through the enforcement of electric reliability standards.81 These
standards are developed by NERC and with oversight by FERC in the United States.82 NERC has
set over 100 reliability standards that cover all timescales of reliability planning.83
Congress gave NERC authority to enforce reliability standards in EPACT05. Per the statute,
NERC has delegated this authority to the Regional Entities shown in Figure A-2. The jurisdiction
for enforcing compliance with reliability standards includes “all users, owners and operators of
the bulk-power system” within the contiguous United States.84
Figure A-2. NERC Regional Entities
Source: CRS, adapted from NERC, “Key Players,” accessed April 17, 2019, https://www.nerc.com/AboutNERC/
keyplayers/pages/default.aspx.
81 16 U.S.C. §824o.
82 Due to the integrated nature of the bulk power system, NERC oversees electric reliability for parts of Canada and
Mexico as well. The electric industry originally established NERC’s predecessor, the National Electric Reliability
Council, in 1968 to coordinate voluntarily reliability efforts. Alaska and Hawaii are exempt from NERC’s jurisdiction.
83 The number of standards in effect changes as NERC modifies and creates standards in response to directives from
Congress or FERC and changing conditions in the electricity system. The current list of standards is available at
https://www.nerc.com/pa/Stand/Pages/AllReliabilityStandards.aspx?jurisdiction=United%20States.
84 16 U.S.C. §824o(b)(1).
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Maintaining Electric Reliability with Wind and Solar Sources
Notes: The multicolored areas represent areas where some entities are members of MRO and some entities
are members of SERC. Alaska and Hawaii are not shown because mandatory reliability standards do not apply in
those states.
Separate from the tasks of setting and enforcing reliability standards is the task of reliably
operating the power system in real time. Balancing authorities carry most of the responsibility for
matching generation levels with electricity demand. Balancing authorities can have different
geographic footprints. RTOs act as balancing authorities and they may have a footprint spanning
multiple states. Other balancing authorities might have a footprint spanning an area within a
single state. Another class of entities with operational responsibilities are reliability coordinators.
A reliability coordinator may operate over larger geographic areas than balancing authorities and
can overrule real-time decisions by balancing authorities to preserve the larger scale power
system reliability. RTOs typically also act as reliability coordinators. NERC has certified 12
reliability coordinators in the United States and several dozen balancing authorities.85
Author Information
Ashley J. Lawson
Analyst in Energy Policy
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
shared staff to congressional committees and Members of Congress. It operates solely at the behest of and
under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other
than public understanding of information that has been provided by CRS to Members of Congress in
connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not
subject to copyright protection in the United States. Any CRS Report may be reproduced and distributed in
its entirety without permission from CRS. However, as a CRS Report may include copyrighted images or
material from a third party, you may need to obtain the permission of the copyright holder if you wish to
copy or otherwise use copyrighted material.
85 NERC, “Reliability Coordinators,” updated July 2022, https://www.nerc.com/pa/rrm/TLR/Pages/Reliability-
Coordinators.aspx, and NERC, NERC Balancing Authority Areas (As of October 2019), https://www.nerc.com/
AboutNERC/keyplayers/PublishingImages/BA%20Bubble%20Map%2020191106.tif.
Congressional Research Service
R45764 · VERSION 3 · UPDATED
22
This is a copy of a public record, reproduced as it was published. It is not legal advice, and it may not be the version a court would rely on. Check the official source before you cite it.