Winter weather extremes determine the cost and adequacy of a zero-emission US power system

Power systems are planned for summer heat waves and, as heating electrifies, winter cold snaps. We ask which weather extremes set the cost of a wind-and-solar grid,and whether the planning weather year matters. In a capacity-expansion model of the United States in 2050, we solve 26 emission caps, re-optimize zero-emission systems on seven weather years, and test selected portfolios under other years. With expanding wind and solar, the heat-wave week never has the highest average price. The highest-priced week falls in winter in six of seven years, at 5.9 to 9.6 times the annual price, and moves between years. These winter weeks are warmer than the coldest week, with unusually low combined wind and solar output. Planning on the mildest winter leaves up to 2.1% of annual energy unserved under other years; adding difficult weeks cuts unserved energy by 99.6% for 8% more cost.