In a first last month, renewables supplied more power to the U.S. than natural gas, a milestone in the shift to clean energy. However, rising power demand is complicating the transition away from fossil fuels by extending the lives of many aging coal power plants.

Together, renewables — including solar, wind, hydropower, and bioenergy — were the biggest source of U.S. electricity in March, according to data from the think tank Ember. Along with nuclear power, they supplied more than half of U.S. power.

The new high watermark for renewables is a reflection both of the rapid buildout of wind and solar and the seasonal drop in power demand. Mild spring weather usually blunts the need for both heating and cooling, and as demand for electricity drops, coal and gas power ramp down. Last month, fossil fuels generated less electricity than in any March going back at least 25 years, while renewables generated more, according to Ember.

Solar, wind, and batteries will account for 93 percent of power capacity added to the grid this year, according to the Energy Information Administration, but rapidly rising power demand is giving a lifeline to fossil fuels. Tech companies are installing natural gas generators at new data centers, while grid operators are delaying the retirements of aging coal power plants.

Nine coal power plants that were set for retirement last year have had their operating lives extended, including five in response to emergency orders from the Department of Energy. Only four coal power plants retired generators last year, according to the EIA. The total coal capacity retired last year was the lowest in 15 years.

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