French and German day-ahead electricity prices jumped more than 20% after Europe's fifth heatwave of the summer tightened supply and lifted cooling demand, with LSEG data for Aug. 11 showing France at 142.50 euros per megawatt-hour and Germany at 138.50 euros. EDF expects 7.3 gigawatts of French nuclear capacity, about 12% of the fleet, to be constrained during peak demand on Aug. 12 as high river-water temperatures and lower water levels force reactors to cut output or shut down under environmental rules. Germany is also facing a sharp drop in wind generation, with LSEG forecasting output to fall by 8.1 GW to 4.7 GW, around 60% below seasonal norms, leaving more demand to be met by gas-fired plants at a time when concerns around the Strait of Hormuz are pressuring gas costs. Analysts expect the combination of tighter supply and higher use of air conditioning and fans, with Météo-France forecasting 35C to 39C temperatures on Aug. 13 and 14, to keep volatility elevated across European power markets and spill into neighboring markets including the UK.