The cost of generating electricity from wind and sunlight has fallen faster than almost anyone predicted, but generation was never the whole problem. Demand peaks on still evenings after dark, precisely when these sources produce least. Batteries can bridge a gap of a few hours and are now being installed at scale, yet they remain expensive for the longer lulls that arrive when a weather system stalls over a region for a week. Grid operators therefore keep gas plants on standby for a small number of hours each year — a solution that is costly and that erodes the very emissions savings it exists to protect.