The California Independent System Operator (CAISO) has accelerated curtailments of solar-powered generation over the past few years as the number of renewable energy projects coming online has outpaced increases in transmission.
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 (dark-blue sections of bars in graphic below), when the power lines don’t have enough capacity to deliver the available energy. Curtailments can also occur due to oversupply (light-blue sections of bars), when generation exceeds customer demand.