Volume of curtailed renewable generation in California has surged over the past two years

The U.S. state of California ranks among the leaders in renewable energy development, not only among U.S. states, particularly when it comes to solar power plants. But further growth in installed capacity from these sources also has a downside: the volume of electricity they have been unable to supply to the grid has increased every year. Last year alone, the volume of curtailed generation rose by almost two-thirds, according to the local grid operator. Nearly all of the curtailed generation came from solar power plants.

In recent years, particularly over the past five, California has seen an increase in curtailed generation from renewable sources that could theoretically have supplied the grid. According to the U.S. government agency, the U.S. Energy Information Administration (EIA), generation is curtailed either because wholesale prices fall or at the direction of the local grid operator.

According to the California Independent System Operator (CAISO), which operates California’s grid, the volume of curtailed generation from solar and wind power plants reached 2.4 TWh last year.

CAISO reported a 63% increase compared with 2021. Between January and September this year, the volume of curtailed generation had already reached 2.3 TWh. Because photovoltaic generation is seasonal, the increase is most evident in the spring and summer months, when this year’s peak volume of curtailed generation had already reached more than twice the levels seen in 2020 and 2021.

In California, curtailment mainly affects solar power plants, which accounted for 95% of the total last year and 94% during the first seven months of this year, according to CAISO. The most curtailment takes place in California in the spring, when solar generation is high and electricity consumption is relatively low after winter and before the start of summer, when demand for electricity rises with the intensive use of air conditioning.

Curtailment is generally caused by one of two factors: low electricity demand or, in most cases, limited grid transmission capacity. Electricity grids, after all, were generally not designed in the past to handle the intermittent output of renewable sources.

Renewable generation is curtailed beyond California

Like California, many other U.S. states and countries around the world have experience with curtailing renewable generation. Texas, for example, also has to significantly curtail solar generation. Last year, the local grid operator had to reduce wind farm output by 5%, while solar farm output was cut by an average of 9%.

Renewable generation has also been curtailed in neighboring Germany in recent years. Here, too, the primary cause is insufficient grid transmission capacity.