Danube Low Forces Hungarian Nuclear Curtailments as EU Solar Hits Summer Record
The Danube's all-time low forced reactor curtailments in late July while Ember data show solar supplied a record 25% of EU power in June.
The Danube fell to an all-time low on 2026-07-29, forcing Hungarian nuclear reactors to curtail output as river temperatures pressed against legal discharge limits, TechTimes reported. Hungarian environmental law bars discharge water temperature from exceeding 30°C within 500 metres of a plant's warm-water outflow channel. A ministerial exemption can allow a brief rise above that threshold in exceptional circumstances, but it does not remove the underlying physics.6
Solar supplied a record 25% of EU electricity in June 2026, making it the bloc's top power source for two consecutive months, per new data from think tank Ember. Five years ago, solar delivered around 10% of EU power even in the sunniest months — a gap explained entirely by capacity additions, not improvements in solar physics.5
Spain and Germany each drew more than one-third of their electricity from solar in June. Hungary, whose reactors were being limited by the same drought conditions, sourced close to half its power from solar in the same month. Running a drought-resilient grid on any single technology is difficult; Hungary demonstrated both sides of that problem simultaneously.5
Britain showed a different pattern. The National Energy System Operator issued a rare overnight warning on 2026-07-09 that extreme temperatures could hit electricity supplies, with household use of fans and air conditioning placing record strain on the system.4 A second margin warning came on 2026-06-24, when NESO flagged roughly 1.3 GW unavailable due to constraints.3
Still air compounded the problem. Wind provided only 13.9% of British electricity demand on 2026-06-23, equal to around 4.32 GW, against a year-long average of 35.2%, or 10.84 GW, Energy Voice reported. Heatwaves arrive with high pressure. The same atmospheric conditions that push temperatures up suppress wind across large areas at the same time.3
Solar carries its own heat penalty. A 2024 paper shows that for each degree warmer, solar panels can lose 0.5% in efficiency, Energy Voice reported; when panel temperature rises 50% above its operating reference, output drops by more than 30%. Peak irradiance and peak heat arrive simultaneously, and the efficiency loss is worst at precisely those hours.3
Yet the EU aggregate held. The installed base has grown fast enough that degraded per-panel output still produced record regional volumes in June. The 25% share recorded that month compared with roughly 10% five years earlier is a function of how much capacity has been added across the continent.5
Britain's nuclear trajectory shows why grid margins have grown tighter. UK nuclear generation in 2025 ran at roughly half the output recorded in 2015, Energy Voice reported. Renewables reached 52.5% of British generation in 2025, the second consecutive year above that threshold — but variable capacity replacing firm baseload shifts the risk profile during high-demand, low-wind, heat-stressed hours.1
Experts told Montel that battery storage paired with solar can absorb some of that stress. During a previous heatwave, solar was cited as the resource that most directly prevented deeper supply shortfalls. Deployment at the scale needed to backstop multi-gigawatt shortfalls remains incomplete across most European systems.2
The summer is not over. Hungary drew close to half its power from solar in June while its nuclear reactors faced curtailment from the same drought draining the Danube. How far river levels fall through the remaining weeks of August, and how long ambient temperatures hold riverside discharge against statutory limits, is the live risk for central European baseload.6,5