Sunrun and Tesla report 580 MW from 140,000 home batteries, while the California bill that would let those batteries count as capacity sits unsigned

On 9 September 2026, with a heat wave pushing California's evening demand up, Sunrun and Tesla drew on more than 140,000 home batteries and pushed a combined peak of more than 580 megawatts into the grid for about three hours. The two companies announced the figure jointly on 21 September. Tesla accounts for 517 MW of it, across roughly 110,000 Powerwalls. Sunrun says it manages 55 per cent of those Powerwalls plus more than 30,000 batteries from other manufacturers, which puts its own share at approximately 383 MW. Sunrun's chief executive Mary Powell said the company's batteries are "operating at a scale larger than many peaker power plants combined."

That comparison is the interesting part, and it is worth being precise about what it does and does not establish. 580 MW is genuinely peaker scale; a single gas peaking unit in California typically sits somewhere between 50 and 300 MW. But the power did not reach the grid through a market that treats these batteries as generation. It came through two discretionary state programmes: the California Energy Commission's Demand Side Grid Support scheme, which the companies say was triggered when the California Independent System Operator's day ahead locational marginal price passed 200 dollars per megawatt hour, and the California Public Utilities Commission's Emergency Load Reduction Program. Both are, in structure, arrangements for calling on customer equipment when the system is in trouble. Neither is a capacity product. A gas peaker gets paid for being there whether or not it runs. A home battery, under these programmes, gets paid for running.

That distinction is not academic this week. SB 913, introduced by Senator Josh Becker, would require the CPUC to update its resource adequacy rules so that aggregated distributed energy resources can be counted against the same reliability requirement that justifies procuring conventional plant. The bill was enrolled on 30 August 2026. As of 27 September the legislature's own history recorded no action by the Governor, and his published legislative update of 20 September listed signings and vetoes without mentioning it. His deadline to sign or veto bills from the 2025 to 2026 session is 30 September 2026; bills he does not act on become law without a signature. So the largest residential dispatch anyone has claimed happened three weeks before the decision about whether such dispatches can ever be more than an emergency favour.

The honest caveat is that every number above comes from the two companies that benefit from it. The press release cites the CAISO day ahead price as the programme trigger, but it does not cite CAISO or the Energy Commission as confirming the 580 MW, and it does not claim they have. Settlement data for the 9 September events would settle it, and until that is published the figure should be read as a vendor claim rather than a measurement. The same applies to the "largest in history" framing: there is no public register of comparable events against which it could be tested.

What is not in doubt is the ownership picture, and that is why this belongs here rather than in a reliability roundup. The 140,000 batteries were bought or leased by households and businesses. They sit behind their meters. They were aggregated by two companies with no distribution franchise and no obligation to serve. For three hours they did work that the grid has historically answered by building something central, and the question the arithmetic cannot settle is who is paid for that work, on what terms, and whether the payment survives the next budget cycle. A programme that pays when the state declares trouble makes the edge a reserve. A resource adequacy rule that counts the same megawatts makes it an asset. One of those is a market and the other is a favour, and which one California chooses is a signature away.