California VPPs hit record during heatwave as bill sits on Governor’s desk

If you have a home battery in California, it may already be doing more than backing up your house during outages. On September 9, during a heat wave, about 110,000 home batteries statewide discharged together at a peak of more than 580 megawatts, the largest event of its kind on record. Tesla Powerwalls made up most of that, with Sunrun operating a little over half of the participating Powerwalls plus more than 30,000 batteries from other brands. The dispatch was triggered by a state program, the Demand Side Grid Support program, which calls on batteries when wholesale electricity prices spike above a set threshold. The next evening, Southern California utility Southern California Edison ran its own three-hour call, and the two events together showed the batteries could deliver over 720 megawatts if triggered on the same night.

This kind of setup, where many home batteries act together as a "virtual power plant," lets utilities lean on stored home power instead of firing up gas plants during the highest-demand hours. An earlier statewide test in July 2025 found the approach could save up to $206 million by avoiding gas peaker plant use during the priciest hours of the year.

The timing matters because two California bills, SB 913 and SB 905, are now awaiting the Governor's signature. They would set rules for valuing and counting home batteries toward the state's grid planning, and measure how much of the power grid's capacity actually gets used. Last year, the Governor vetoed similar bills and cut funding for the program behind the September dispatch, so the outcome is uncertain.