CAISO, the California Independent System Operator, the grid operator for most of the state, is increasingly curtailing solar and wind powered electricity generation as it balances supply and demand in the face of rapid renewables capacity growth.
The output of wind and solar generators is reduced either through price signals or, rarely, through an order to reduce output during periods of congestion, when power lines do not have enough capacity to deliver available energy, and oversupply, when generation exceeds customer electricity demand.
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. Solar accounted for 93% of all the energy curtailed in CAISO in 2024. CAISO curtailed the most solar in the spring, when solar output was relatively high and electricity demand was relatively low, because moderate spring temperatures meant lower demand for space heating or air conditioning. 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 had grown to 28.2 GW.
CAISO also curtails solar generation to leave room for natural gas generation. A certain amount of natural gas generation must stay online to comply with North American Electric Reliability Corporation (NERC) reliability standards and for the evening demand ramp up.
CAISO is trying to reduce curtailments in several ways: by trading with neighbouring balancing authorities to try to sell excess solar and wind power, by incorporating battery storage into ancillary services, energy, and capacity markets, by including curtailment reduction in transmission planning, and by promoting the addition of flexible resources that can rapidly respond to sudden increases and decreases in demand.
In addition, starting this year, companies are planning to use excess renewable energy to make hydrogen, some of which will be stored and mixed with natural gas for summer generation at the Intermountain Power Project’s new facility scheduled to come online in July.
