Tesla and Sunrun’s home batteries sent a record 580 MW to California’s grid

Tesla and Sunrun pushed a record 580 megawatts from home batteries into California’s grid on September 9, drawing on more than 140,000 residential systems during a heat wave that pushed wholesale power prices past $200 per megawatt-hour. Sunrun said the dispatch was the largest residential virtual power plant event on record, with roughly 110,000 Tesla Powerwalls supplying 517 megawatts of the total.

Sunrun operates 55 percent of the Powerwalls that took part, while batteries from other manufacturers contributed the remaining 63 megawatts. The company followed up the next day with an additional 140-plus megawatts at the direct request of Southern California Edison, one of the state’s three big investor-owned utilities, bringing the two-day total north of 720 megawatts had both events run at once rather than a day apart.

Electrek, which first reported the dispatch figures on September 21, described the event as the largest residential virtual power plant dispatch in the state’s history, a characterization Sunrun’s own numbers support: no prior aggregation of home batteries in California has been reported at anywhere near 580 megawatts in a single event.

The two dispatches were triggered by different mechanisms, one automatic and one manual, on consecutive days of the same heat wave.

Two State Programs Triggered the Same Week

The September 9 event was activated automatically under the California Energy Commission’s Demand Side Grid Support program, which pays for standby capacity that kicks in once the day-ahead wholesale price crosses a set threshold — in this case, $200 per megawatt-hour. The September 10 dispatch instead ran through the California Public Utilities Commission’s Emergency Load Reduction Program, under which a utility calls directly on enrolled batteries when it needs relief on its own system rather than waiting for a statewide price signal.

Sunrun CEO Mary Powell framed the scale of the combined events in a company statement announcing the results. “Sunrun’s distributed home batteries are operating at a scale larger than many peaker power plants combined,” Powell said, comparing the fleet’s output directly to the natural-gas peaker plants that utilities typically bring online during demand spikes.

Enough Power for Every Household in Sacramento County

Sunrun’s own comparison for the September 9 dispatch: 580 megawatts is roughly enough to supply every household in Sacramento County during peak evening hours. That is a useful yardstick because it converts a grid-operator’s unit, the megawatt, into something closer to what a single dispatch event actually replaces on a hot evening when demand peaks right as rooftop solar production is falling off.

The comparison also illustrates why utilities have leaned harder on these programs in recent summers. A single natural-gas peaker plant typically runs in the 50 to 250 megawatt range, meaning the September 9 event alone stood in for several peaker plants running at once, without a new power plant ever being built.

A Bigger Buildout Already Under Way

Sunrun, Tesla, and Renew Home are separately working on a much larger project: a 16.8-gigawatt virtual power plant program announced in June, aimed partly at absorbing demand from AI data centers and running through Pacific Gas & Electric’s territory. That project remains in early stages and is not the same fleet that dispatched on September 9, but it points to where the residential-battery-as-power-plant model is headed if the September events prove replicable rather than a one-off.

None of the three companies has said how large a share of that 16.8-gigawatt target is expected to come from batteries already sitting in customers’ garages versus new installations still to be sold and installed over the coming years, a distinction that matters for how quickly the bigger number could ever be tested the way the September 9 event was. Getting there would require roughly 29 times the battery capacity that dispatched on September 9, spread across a customer base that has to opt in, install hardware, and agree to let a utility or aggregator draw down their battery during a crunch.

An independent economic analysis from the Brattle Group, a consulting firm, estimated that scaling programs like these across the state could deliver net savings of up to $206 million to California ratepayers by 2028, largely by avoiding the cost of building and maintaining new peaker capacity that would otherwise sit idle most of the year.

Whether the September dispatch becomes a template depends on how often California’s grid operator needs it. Heat waves that push wholesale prices past $200 per megawatt-hour are becoming a regular late-summer occurrence in the state, and the DSGS program was specifically designed to activate automatically when that threshold is crossed, rather than requiring a manual call from grid operators watching conditions in real time. Southern California Edison’s decision to go further and request an additional dispatch on September 10, after the automatic DSGS trigger had already fired the day before, suggests utilities are treating these residential battery fleets less like a backstop reserved for the worst hours and more like a resource they can call on repeatedly across a multi-day heat event.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview