Simultaneously, electric heaters created a powerful surge in demand. About 61% of Texans rely on electric heat, mostly low-efficiency resistance heat, in poorly insulated homes — a seemingly sensible choice in a warm climate with cheap electricity, where home heating is often unnecessary. The demand surge caused by the cold came to about 20 GW or one-third of the winter peak. ERCOT based its worst-case analysis on a 2011 winter storm, the most severe cold-weather event in Texas in twenty years. The 2021 storm was much worse than that of 2011. It created an unexpected and unsupportable demand surge.