As temperatures across New England soared above 100 degrees Fahrenheit in recent weeks, solar panels and batteries helped keep air conditioners running while reducing fossil-fuel generation and likely saving consumers more than $20 million.

“Local solar, energy efficiency, and other clean energy resources helped make the power grid more reliable and more affordable for consumers,” said Jamie Dickerson, senior director of clean energy and climate programs at the Acadia Center, a regional nonprofit that analyzed clean energy’s financial benefits during the recent heat wave.

On June 24, as temperatures in the Northeast hit their highest levels so far this year, demand on the New England grid approached maximum capacity, climbing even higher than forecast. Then, unexpected outages at power plants reduced available generation by more than 1 gigawatt. As pressure increased, grid operator ISO New England made sure the power kept flowing by reducing exports to other regions, arranging for imports from neighboring areas, and tapping into reserve resources.

At the same time, rooftop and other ​“behind-the-meter” solar panels throughout the region, plus Vermont’s network of thousands of batteries, supplied several gigawatts of needed power, reducing demand on an already-strained system and saving customers millions of dollars. It was a demonstration, supporters say, of the way clean energy and battery storage can make the grid less carbon-intensive and more resilient, adaptable, and affordable as climate change drives increased extreme weather events.

“As we see more extremes, the region still will need to pursue an even more robust and diverse fleet of clean energy resources,” Dickerson said. ​“The power grid was not built for climate change.”

On June 24, behind-the-meter solar made up as much as 22% of the power being used in New England at any given time, according to the Acadia Center. At 3:40 p.m., total demand peaked at 28.5 GW, of which 4.4 GW was met by solar installed by homeowners, businesses, and other institutions.

As wholesale power prices surpassed $1,000 per megawatt-hour, this avoided consumption from the grid saved consumers at least $8.2 million, according to the Acadia Center.

This estimate, however, is conservative, Dickerson said. He and his colleagues also did a more rigorous analysis accounting for the fact that solar suppresses wholesale energy prices by reducing overall demand on the system. By these calculations, the true savings for consumers actually topped $19 million, and even that seems low, Dickerson said.

In Vermont, the state’s largest utility also relieved some of the pressure on the grid by deploying its widespread network of residential and EV batteries. That could save its customers some $3 million by eliminating the utility’s need to buy expensive power from the grid and reducing fees tied to peak demand.

“Green Mountain Power has proven that by making these upfront investments in batteries, you can save ratepayers money,” said Peter Sterling, executive director of trade association Renewable Energy Vermont. ​“It’s something I think is replicable by other utilities in the country.”

Green Mountain Power’s system of thousands of batteries is what is often called a ​“virtual power plant” — a collection of geographically distributed resources like residential batteries, electric vehicles, solar panels, and wind turbines that can work together to supply power to the grid and or reduce demand. In Vermont, Green Mountain Power’s virtual power plant is its largest dispatchable resource, spokesperson Kristin Carlson said. The 72-MW system includes batteries from 5,000 customers, electric school bus batteries, and a mobile, utility-scale battery on wheels.

The network began in 2015 with the construction of a 3.4-megawatt-hour storage facility at a solar field in Rutland, Vermont. Two years later, the utility launched a modest pilot program offering Tesla’s Powerwall batteries to 20 customers, followed in 2018 by a pilot that paid customers to share their battery capacity during high-demand times. In 2022, a partnership with South Burlington’s school district linked electric school buses to the system, and in 2023, state regulators lifted an annual cap on new enrollments it had imposed on a Green Mountain Power program that leases batteries to households. The number of customers with home batteries has since grown by 72%.

“We’ve had a really dramatic expansion,” Carlson said. ​“It is growing by leaps and bounds.”

The network saved consumers money during the heat wave by avoiding the need to buy power at the high prices the market reached that day, but also by helping to lower the ​“capacity fees” charged by ISO New England. These charges are determined by the one hour of highest demand on the grid all year, and then allocated to each utility based on their contributions to that peak. By pulling power from batteries rather than just the grid, Green Mountain Power lowered its part of the peak.

If the afternoon of June 24 remains the time of peak demand for 2025, Green Mountain Power’s 275,000 customers will save about $3 million in total and avoided power purchases, the utility calculated. Looking ahead, more hot weather and further expansion of the utility’s virtual power plant will likely continue to put money back in customers’ pockets, Sterling said: ​“When you play that out over many years, that’s real savings to ratepayers.”