France’s solar power supports the grid as 13 nuclear reactors slow during a heatwave

On the morning of August 14, France’s nuclear fleet was producing around 32 GW, far below the approximately 45 GW seen on a typical day in March. Heat and drought had forced 13 reactors offline, equal to 20.4% of the country’s nuclear generating capacity, while EDF warned that weather conditions could cause a maximum capacity loss of 12,474 MW between August 14 and September 1.

Solar power was running in the opposite direction. For several weeks, it had supplied up to 35% of France’s electricity generation mix. Ember found that solar generation was 17% above daily normal levels in France and Hungary during the heatwaves of late June and July, while output was 5% above normal in Spain and Italy. The effect is straightforward: sunlight is strongest during the same hot afternoons when air-conditioning increases demand, helping compensate for weaker nuclear and thermal generation.

France was not alone. Five gas-fired plants in the United Kingdom reduced production, low water levels threatened thermal plants in northeastern Italy, and Hungary’s Paks nuclear plant sharply cut output as the Danube fell to exceptionally low levels. Coal plants in Serbia and Poland also scaled back. Hydropower faced similar strain: seven of the European Union’s eight largest producers recorded generation below their five-year averages.

The harder test begins after sunset. Photovoltaic generation falls to zero while demand can remain high, forcing the grid to rely more heavily on dispatchable sources such as gas-fired plants. During the heatwaves in late June and early July, average daily electricity prices increased by 44% in France compared with the preceding weeks, and Ember reported notable spikes in the early evening.

So what changes in practice? Solar is already helping the grid through the hottest part of the day, when several conventional power sources are under pressure. Batteries could extend that benefit into the evening by storing daytime solar electricity and releasing it when demand remains high, potentially reducing reliance on the most expensive thermal generation. The figures come from EDF, RTE and Ember; the August heatwave was not yet included in Ember’s study.