NESO (the National Energy System Operator) issued three Electricity Margin Notices in a single summer for the first time on record — on 24 and 26 June and again on 8 July — as heatwave conditions pushed evening electricity demand up just as solar output was falling and wind generation stayed low. One notice pointed to an estimated shortfall of around 1.2 gigawatts during the 6:30pm–10:30pm peak, with temperatures near 36°C across parts of the country.
A single Electricity Margin Notice doesn't mean blackouts are imminent — it's a routine tool NESO uses to ask generators and large industrial users for more spare capacity before the system gets genuinely tight. But the fact that summer notices, historically almost unheard of, have now happened three times in one year is a signal worth paying attention to. Part of the pressure is coming from across the Channel: France's nuclear fleet, which the UK normally imports from via interconnector, has itself been running under reduced capacity during the same heatwaves.