More than 140,000 home batteries across California dispatched more than 580 megawatts (MW) of peak power to the state's grid on the evening of September 9, according to Sunrun and Tesla, which operate the fleets. The companies say it was the largest residential virtual power plant dispatch on record, Solar Builder reported.

The figures come from the two companies, which announced the results on September 21 and have a commercial interest in the outcome. No independent grid operator data confirming the total appeared in the materials reviewed for this article. Still, the event offers a concrete look at an idea that is reshaping how some utilities plan for peak demand.

For California households, the stakes are practical. Heat waves push electricity demand and wholesale prices up in the evening, when solar output fades. If batteries already sitting in garages can supply some of that power, the state may need fewer expensive backup plants that run only a few hours a year.

A Power Plant Made of Garages

A virtual power plant, or VPP, is a network of small energy devices, such as home batteries, solar panels and smart thermostats, that software coordinates to act like a single power plant. Each home battery on its own is small. Pooled together and discharged at the same moment, thousands of them can supply as much power as a conventional generator.

Most of these batteries are charged by rooftop solar during the day. When a grid program calls for help, the companies' software signals enrolled batteries to discharge for a set window, easing demand on the wider system. Participants are compensated under the terms of the programs they join.

By the companies' account, Tesla Powerwalls supplied 517 MW of the total, and about 30,000 batteries from other manufacturers in Sunrun's fleet added 63 MW. About 110,000 Powerwalls took part, and Sunrun says it owns and operates 55% of them. The dispatch ran for a three-hour evening window. Sunrun and Tesla said the output was enough to power every household in Sacramento County during peak hours, an equivalence based on their own calculation.

Inside the September 9 Dispatch

The batteries responded through two statewide programs: the California Energy Commission's Demand Side Grid Support program and the California Public Utilities Commission's Emergency Load Reduction Program. The state program triggered automatically once the day-ahead price crossed $200 per megawatt-hour, while utilities called the second program directly as the heat wave peaked, Electrek reported.

The following evening, Southern California Edison requested its own three-hour dispatch, and the companies said they delivered more than 140 MW. Had both calls come on the same night, Sunrun and Tesla said, their fleets could have delivered more than 720 MW.

Sunrun CEO Mary Powell said the company's distributed home batteries are "operating at a scale larger than many peaker power plants." She added that the dispatch shows the benefits of distributed energy going beyond individual households by helping control costs and reducing the need for new poles and wires.

Checking the Numbers

The record builds on an earlier test. During a statewide event on July 29, 2025, more than 100,000 batteries sustained 539 MW for two hours, with Tesla units supplying 459 MW of that. Solar Builder reported that research firm The Brattle Group finds these Sunrun and Tesla programs could provide up to $206 million in energy savings for California residents by 2028, benefiting all grid-connected customers, with participants also earning direct compensation.

There are limits to keep in mind. The 580 MW figure is a peak, not an average across the full three hours, and the companies have not published hour-by-hour data. A battery fleet can also only discharge what it has stored, so a VPP suits evening peaks lasting a few hours rather than multi-day emergencies. How much grid operators can count on residential fleets in long-term planning depends on consistent performance across many events, not on a single record.

The policy context is shifting too. The California Legislature has sent the governor bills that would let aggregated home batteries count toward resource adequacy, a year after three virtual power plant and load-flexibility bills were vetoed, as trade outlet Energy Tech reported alongside coverage of the dispatch.

Homeowners, Backup Power and Bills

The households most directly affected are the roughly 140,000 that took part, whose batteries discharged for part of the evening. Program rules generally determine how much charge remains for home backup, so participants should review their enrollment terms if outage protection is a priority, particularly during wildfire season in areas subject to power shutoffs.

For other ratepayers, the potential benefit is indirect. If VPPs reduce reliance on the most expensive peak generation and delay some grid upgrades, they could help restrain costs over time. But the size of any bill impact is not yet established by independent analysis, and California rates depend on many factors beyond peak power.

Homeowners with batteries who want to participate can ask their installer or utility about eligible programs, compensation, and how often dispatches occur. Sunrun, Tesla and Renew Home announced in June a plan to unlock more than 16.8 gigawatts of capacity from home batteries and other distributed devices, a sign that more programs are likely.

Sunrun and Tesla report that 140,000 California home batteries delivered a record 580 MW on September 9, showing how a virtual power plant can supply power like a conventional plant during a heat wave. The figures are company-reported, and independent data on performance and bill savings will determine how much the grid can rely on such fleets.

What Readers Want to Know

What happened on September 9?

Sunrun and Tesla say more than 140,000 home batteries in California dispatched more than 580 MW of peak power to the grid during a three-hour evening window amid a heat wave.

What is a virtual power plant?

A network of small devices, such as home batteries and solar panels, coordinated by software to act like a single power plant when the grid needs extra supply.

Who confirmed the record?

The record claim and figures come from Sunrun and Tesla. The materials reviewed did not include independent grid operator data confirming the total.

Why was the dispatch triggered?

Heat drove up demand and prices. A state program activated automatically when the day-ahead price crossed $200 per megawatt-hour, and utilities called a second program.

Do participants lose backup power?

Program terms set how much charge is used. Homeowners who rely on batteries for outages should review their enrollment rules.

Will this lower electricity bills?

Possibly over time. The Brattle Group estimate of up to $206 million in savings by 2028 was reported alongside the companies' announcement, and independent evidence on bill impacts is still limited.