Anyone notice that this past winter was particularly cold in the eastern United States? Plenty of families in the Northeast certainly did, and they turned up their thermostats for weeks at a time to stay warm.

Now, a new report from the U.S. Department of Energy reveals a surprising fact about the performance of America’s power grid during the coldest parts of the recent winter: As households reached for the thermostat, it was coal-fired power plants that saved the day. In many parts of the country, these reliable plants stepped up to cover 55 percent of incremental daily power generation. And so, when families most needed to warm their homes, it was old king coal—not natural gas or wind turbines—that made it possible.