National Grid, the company responsible for keeping Britain’s lights on, declared last month that the margin between electricity supply and demand over the coming winter would be “tight but manageable.” Three weeks later, National Grid had to issue its first call in more than five years for emergency supplies to keep the grid stable. At midday on Nov. 4, it asked for an extra 500 megawatts for the 6:30 to 8.30 evening peak.

It was an unseasonably mild and windless day. A couple of coal plants nearing the end of their useful...