Millions of Texans were left without electricity Monday as a rare winter storm boosted demand and crimped supplies, demonstrating that California isn't the only state with a power grid vulnerable to extreme weather conditions.

The Electric Reliability Council of Texas, which manages the state's electricity grid, began calling for rotating outages overnight on Sunday to avoid widespread blackouts that can occur when demand exceeds supply. But the severe power shortages forced companies to curtail power beyond short rolling blackouts, with many customers losing electricity for much of the day.

Ercot estimated that about 2 million homes were without power on Monday and cautioned that the outages would likely continue through Tuesday. By Monday evening, it said power had been restored to about 500,000.

The imbalance occurred as residents cranked their thermostats amid record-breaking lows in some areas of the state, causing electricity demand to surge amid a precipitous drop in generating capacity. The grid operator said it lost about 34,000 megawatts of supply as freezing temperatures forced natural-gas- and coal-fired power plants offline in quick succession. The weather also reduced natural-gas supplies to power plants and caused wind turbines in West Texas to freeze.