Millions of Texas households are still without power in a brutally cold winter storm. What went wrong?

The entire state of Texas was under a winter storm warning on Monday, with snow falling throughout the state and single-digit temperatures as far south as Austin and San Antonio. As Texans turned up their heaters on Sunday night, the freezing temperatures took down several power generation plants, prompting the Electric Reliability Council of Texas (ERCOT) — which manages the state's uniquely independent power grid — to order rolling blackouts at 1:25 a.m. Monday, rather than risk a collapse of the entire grid.

More than 2 million customers lost electricity by Monday morning, and by Monday night, 4.2 million Texas customers were without power as the temperatures hit record lows, according to PowerOutage.us, a site that tracks power outages nationwide. Texas utilities are warning those households they may not get power until Tuesday afternoon or evening, right before a second storm is forecast to hit. What went wrong?

First, Texas isn't set up for extreme cold. "The electricity grid was designed to be in high demand during the summer, when Texans crank their air conditioning at home," The Texas Tribune explains. "Some of the energy sources that power the grid during the summer are offline during the winter." Wholesale power prices on the largely deregulated Texas market shot up over the weekend, prompting power generators to maximize their output, The Wall Street Journal reports. Then non-weatherized wind turbines started freezing and natural gas and coal plants tripped offline.

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"This weather event, it's really unprecedented," ERCOT senior director of system operations Dan Woodfin said Monday, pointing to the 1940s as the last time Texas faced this combination of Arctic temperatures and wind chills. "Most of the plants that went offline during evening and morning today were fueled" by coal, gas, or nuclear power, he added. About 40 percent of Texas electricity comes from natural gas-fired plants, followed by wind turbines (23 percent), coal (18 percent), and nuclear power (11 percent), the Journal reports, citing ERCOT's 2020 data.

With 30 gigawatts of power generation knocked offline — enough to power almost 6 million homes — the rolling blackouts got stuck. The blackouts were supposed to last less than an hour at each household, but "local utilities kept power on to neighborhoods with hospitals, fire stations, and water-treatment plants," the Journal reports. "There was so little extra power that utilities couldn't rotate the blackouts among neighborhoods that didn't have critical infrastructure, leaving some homes without power for more than 12 hours."