Grid operators are tasked with balancing electricity supply and demand to ensure a stable and safe electric system. When power generation significantly exceeds demand, grid operators must intentionally reduce production in a process called curtailment.
In California, curtailment of solar power has been rising for years, reaching new highs in 2024, said the Energy Information Administration (EIA).
EIA said solar accounts for 93% of all curtailment in the California Independent System Operator (CAISO) region, which services most of the state. In 2024, CAISO curtailed 3.4 million MWh of utility-scale wind and solar output, a 29% increase from the amount of electricity curtailed in 2023.
Curtailment is often dictated by price signals on the grid, said EIA. More rarely, grid operators issue orders to stop output during times of grid congestion or energy oversupply.
CAISO also orders solar curtailment leave room for natural gas generation. A certain amount of natural gas generation must stay online throughout the day to comply with North American Electric Reliability Corporation (NERC) reliability standards and to have generation online in time to ramp up in the evening hours.
CAISO is trying to reduce curtailments by:
- Trading with neighboring balancing authorities to try to sell excess solar and wind power
- Incorporating battery storage into ancillary services, energy, and capacity markets
- Including curtailment reduction in transmission planning
In addition, companies are finding other ways to use excess renewable energy production, like using it to produce hydrogen fuel, some of which will be stored and mixed with natural gas to support summer peak demand events.
(Read: “Can renewables curtailment be rethought as a good thing?”)
CAISO is also cutting curtailment by producing the addition of flexible resources like battery energy storage with 4-to-8-hour dispatch durations. Batteries can charge using excess solar power at midday and then discharge that energy when the sun is going down, providing electricity during hours when it is most needed.
Solar supplies roughly 50% of CAISO’s demand during daylight hours, but solar production ramps down as demand ramps up when people return home from work during the evening and run air conditioning and appliances.
Energy storage can help store and dispatch during these critical hours to smooth out supply and demand peaks and reduce curtailment. Energy storage is already rising to meet the moment, as battery capacity in CAISO increased by 45% in 2024, from 8.0 GW in 2023 to 11.6 GW in 2024, said EIA.
In 2014, a combined 9.7 GW of wind and solar photovoltaic capacity had been built in California. By the end of 2024, that number grew to 28.2 GW. As renewables capacity rises, grid operators will need to continue to pursue solutions to match electricity demand with energy production.
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Curtailment is nuts, why don’t the power companies just build out more batteries.