Estimates of the marginal curtailment rates for solar and wind generation

As the amount of solar and wind generation capacity installed in a region grows, there will increasingly be periods during which a portion of the potential renewable generation will need to be curtailed to maintain the stability of the electric grid. Across markets worldwide, average curtailment rates for wind and solar are generally quite low, often around 3%. However, these low average curtailment rates may overstate how much renewable supply increases as a result of further increases in renewable capacity. Using historical hourly generation and curtailment data from California’s electricity market, we estimate that only 91% of the output supplied by new solar capacity goes towards increasing the state’s renewable supply — with the remaining 9% being discarded in the form of increased curtailments.