May 28, 2025

Solar and wind power curtailments are increasing in California

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The California Independent System Operator (CAISO), the grid operator for most of the state, is increasingly curtailing solar- and wind-powered electricity generation as it balances supply and demand amidst rapid renewables capacity growth.

Grid operators must balance supply and demand to maintain a stable electric system. 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 don’t have enough capacity to deliver available energy
• Oversupply, when generation exceeds customer electricity demand

In 2024, CAISO curtailed 3.4 million megawatthours (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 less demand for space heating or air conditioning.

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In 2014, a combined 9.7 gigawatts (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 throughout the day to comply with North American Electric Reliability Corporation (NERC) reliability standards and to have generation online in time to ramp up in the evening hours.

Solar energy supplies almost half of CAISO’s electricity demand between the hours of 8:00 a.m. and 4:00 p.m., but demand increases in the later evening hours when people come home from work and turn up air conditioners or electric heaters and turn on lights, ovens, computers, and televisions. This need is especially apparent on hot summer evenings after the sun has set and no longer produces solar power overnight.

CAISO is trying to reduce curtailments in several ways:

• Trading with neighboring balancing authorities to try to sell excess solar and wind power
• Incorporating battery storage into ancillary services, energy, and capacity markets
• Including curtailment reduction in transmission planning

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.

The Western Energy Imbalance Market (WEIM) is a real-time market that allows participants outside of CAISO to buy and sell energy to balance demand and supply. In 2024, more than 274,000 MWh of curtailments were avoided by trading within the WEIM, equivalent to about 8% of the electricity curtailed that year. The Extended Day-Ahead Market (EDAM) is expected to be operational by May 2026 and will allow CAISO another outlet to sell solar energy.

To further reduce renewable curtailments and increase the stability of the grid, CAISO is promoting the addition of flexible resources that can quickly respond to sudden increases and decreases in demand. Battery storage, recently the key flexible resource to come online, allows some renewable energy to be stored and used 4-8 hours later in the day. Batteries can charge using excess solar power at midday and then discharge that energy when the sun is going down, providing electricity during hours when it is most needed. Battery capacity in CAISO increased by 45% in 2024, from 8.0 GW in 2023 to 11.6 GW in 2024 according to our survey of recent and planned capacity changes. However, in the spring, more solar energy than can be used within a day is often produced. Without more transmission capacity or a long-term storage solution, high curtailments during this time of year can still occur.

Principal contributor: Lori Aniti