California’s Biggest Battery Test Ever Just Changed the Game

Date: August 04, 2025 Author: Paul Doherty

• Future of Energy

At exactly 7:00pm, a massive experiment began. Across the state, thousands of Tesla Powerwalls and Sunrun home batteries kicked into action. These weren’t just helping individual homes—they were working together as one giant, invisible power plant. For two hours, this “virtual power plant” (VPP) delivered 535 megawatts of electricity to the grid. That’s enough to power hundreds of thousands of homes during peak demand.

This wasn’t a blackout. It wasn’t an emergency. It was a test. But it was the largest test of its kind ever done in California—and maybe the world.

What’s a Virtual Power Plant?

A virtual power plant is a network of small energy sources—like home batteries—that work together to support the electric grid. Instead of one big power station, it’s thousands of little ones, all coordinated by software.

In this case, the batteries belonged to electric customers of Pacific Gas and Electric Co. (PG&E), Southern California Edison (SCE), and San Diego Gas & Electric (SDG&E). These customers had signed up for special programs that let their batteries help the grid when needed.

Why Now?

With electric demand surging and climate events worsening, the grid is prone to peaks of extreme stress. That’s why the California Public Utilities Commission (CPUC) and the California Energy Commission (CEC) created programs like the Emergency Load Reduction Program (ELRP) and Demand Side Grid Support (DSGS). These programs reward people and companies for reducing electricity use—or providing backup power—when the grid is stressed.

But before these programs can be fully trusted, they need to be tested. 

The Test: A Quiet Revolution

On July 29, from 7:00pm to 9:00pm, the batteries discharged power to the grid. More than half of the energy came from customers in PG&E’s Northern and Central California service area. The goal? To prove that these batteries can respond quickly, reliably, and in sync.

This wasn’t just a tech demo. It was a real-world rehearsal for when they’ll really be needed (during grid emergencies).

Tesla and Sunrun brought in an independent third-party, Brattle Group, to analyze and assess the impact of the test.

“On peak days, using VPPs to serve CAISO’s net peak could reduce the need to invest in new generation capacity and/or relieve strain on the system associated with the evening load ramp,” the report concluded. 

(The evening load ramp is when solar generation tails off as the sun sets and other generation sources—such as natural gas-fired power plants--ramp up.) Read the full report below. 

Equity in Energy: Batteries Where They’re Needed Most

One of the most powerful parts of this test was the participation of customers from underserved and rural communities and areas more prone to outages. Some are from neighborhoods where power loss due to Enhanced Power Line Safety settings is more frequent, and where backup energy is most needed.

PG&E has made it a priority to support these communities through programs including: 

• The Residential Storage Initiative  helps customers in high fire-threat districts or those with medical needs get free or low-cost backup batteries.

• The Self-Generation Incentive Program (SGIP) provides rebates for installing battery storage, with extra support for low-income and medically vulnerable customers.

• The Power Saver Rewards Program  offers incentives for reducing energy use during peak times.

These programs ensure that battery storage isn’t just a luxury—it’s a lifeline. And during the July 29 test, these customers played a vital role in keeping California’s grid strong. They didn’t just protect their own homes—they helped power the state.

Why It Matters

• Scale: This was the largest behind-the-meter battery test ever in California, coordinated across multiple utilities, agencies, and participating battery service providers and their customers.

• Reliability: It showed that home batteries can be counted on during peak demand.

• Flexibility: These systems can be turned on and off quickly, helping balance the grid.

• People Power: It proved that everyday Californians now play a big role in keeping the lights on.

What’s Next?

This test was just one of many planned. ELRP requires at least 20 hours of battery dispatch each year, and DSGS requires at least one event per month. If no real emergencies happen, test events like this one will continue to make sure everything works as expected.

“VPPs deliver for the grid! This test embodies California's innovation economy by applying digital tools to access and aggregate untapped electric resources - supporting grid reliability and helping manage electricity costs," said CEC Commissioner Andrew McAllister. “We're all connected to the same grid, and small contributions from many individuals add up to hugely important, shared benefits. Even bigger, better things to come!" 

The Big Picture

California is leading the way in clean energy innovation. By turning homes into power plants, the state, it’s utilities and clean-tech partners are building a smarter, more resilient grid—one that can handle heat waves, wildfires, and the growing demand for electricity.

This test wasn’t just about technology. It was about trust. And on July 29, California’s virtual power plant earned a big dose of it.