Curtailments of renewable generation have continued to accelerate in the California Independent System Operator (CAISO) market in recent years, reaching as high as 840,000 MWh in April 2024 (see chart below), as the number of renewable energy projects coming online has outpaced increases in transmission. A report by the Energy Information Administration (EIA) last year estimated that solar generation accounted for more than 90% of California curtailments.
During a curtailment, grid operators like CAISO, which handles the power grid for most of the state, require renewable generators to reduce energy output below the levels they would have otherwise produced. In California, this is mostly due to congestion, when the power lines don’t have enough capacity to deliver the available energy. Curtailments can also occur due to oversupply, when generation exceeds customer demand.
Curtailments in the state are generally seasonal, although the month-to-month differences have been more pronounced in recent years, with the highest number of curtailments tending to come in March, April and May, when longer days allow for greater solar generation while temperatures are generally moderate and cooling demand is less than summertime levels.
