The last two coal-fired power plants in New England are set to close by 2025 and 2028, ending the use of a fossil fuel that supplied electricity to the region for more than 50 years.

The decision to close the Merrimack and Schiller stations, both in New Hampshire, makes New England the second region in the country, after the Pacific Northwest, to stop burning coal.

Environmentalists waged a five-year legal battle against the New Hampshire plants, saying that the owner had discharged warm water from steam turbines into a nearby river without cooling it first to match the natural temperature.

In a settlement reached on Wednesday with the Sierra Club and the Conservative Law Foundation, Granite Shore Power, the owner of the plants, agreed that Schiller would not run after Dec. 31, 2025 and that Merrimack would cease operations no later than June 2028.

“This announcement is the culmination of years of persistence and dedication from so many people across New England,” said Gina McCarthy, a former national climate adviser to President Biden and former administrator of the Environmental Protection Agency during the Obama administration who is now a senior adviser at Bloomberg Philanthropies, which supports efforts to phase out coal.

“I’m wicked proud to live in New England today and be here,” Ms. McCarthy said. “Every day, we’re showing the rest of the country that we will secure our clean energy future without compromising.”

In recent years, the two New Hampshire power plants have operated only intermittently during peak periods.

After shutting down, the plants will be converted to solar farms and battery units that can store electricity generated from offshore wind turbines along the Atlantic Coast, the company said.

“From our earliest days as owners and operators, we have been crystal clear: While our power occasionally is still on during New England’s warmest days and coldest nights, we were firmly committed to transitioning our facilities away from coal and into a newer, cleaner energy future,” Jim Andrews, the chief executive of Granite Shore Power, said in a statement.

Coal use has dropped precipitously in the United States as natural gas and renewable sources like wind and solar have become less expensive. Coal produced about 17 percent of American electricity in 2023. While operators of a few coal-fired plants could delay planned retirements in the next several years because of surging national demand for electricity, analysts say the coal industry is in sharp decline.

The dirtiest fossil fuel, coal accounted for 59 percent of carbon emissions from electricity in 2021, although it generated less than a quarter of the electricity produced in the United States that year, according to the E.P.A.

In addition to the lawsuit and an advocacy campaign, climate activists used another strategy to prompt the closures: forcing the coal plants to compete in the electricity market.

The Conservation Law Foundation, a New England legal advocacy group focused on environmental issues, had for years lobbied the state electricity regulators and lawmakers in New Hampshire to divide the company that owned both the coal power plants and the utilities that distribute the electricity into separate entities. That’s because, under one corporate structure, they could raise rates to cover both generation and transmission costs.

But the foundation bet that once separated from the transmission business, the owner of the power plants would stop burning coal in favor of other, less expensive sources in order to compete with much cheaper renewable energy,

The coal power plants survived “on the backs of ratepayers, despite their inefficiencies,” said Tom Irwin, a vice president at the Conservation Law Foundation. “They are effectively subsidized.”

“Our goal was to expose these plants to the market forces,” Mr. Irwin said.

The foundation’s efforts proved successful in 2018, when the new owner, Granite Shore Power, bought the power plants with the intent to transition coal generators into less polluting operations.