Extreme heat and exceptionally low Danube flows are tightening electricity supply across Southeast Europe, curbing nuclear and hydro generation while increasing reliance on fossil plants and cross-border imports. The constraints have affected multiple countries with generation tied to river conditions.
Hungary’s nuclear outage and solar-driven demand shifts
Hungary’s Paks nuclear plant, with a capacity of 2 GW, was forced to halt generation in early August after low Danube levels restricted cooling. The plant normally supplies as much as 40% of national electricity. Hungary has installed around 8 GW of solar capacity, enabling photovoltaic output to cover as much as 80% of demand during some daytime periods.
After sunset, the system becomes significantly tighter, requiring fossil plants to increase output. This shift also pushes Hungary toward regional imports during evening hours. The timing aligns with cooling demand remaining elevated when solar generation disappears.
Nuclear and thermal impacts at Cernavodă and Kozloduy
Romania has faced similar problems at the Cernavodă nuclear plant, where Danube-related conditions have constrained operations. Bulgaria reduced production at Kozloduy Unit 5 by around 120 MW due to falling Danube levels. Bulgarian authorities have also moved to build a 250-300 metre structure near Kozloduy to protect water availability for the plant.
Serbia’s hydrology and regional baseload reductions
Serbia is affected through both domestic hydrology and tighter regional supply. Danube inflows at the Djerdap hydropower complex have fallen to around 1,400 cubic metres per second, sharply restricting hydroelectric production. Reduced nuclear availability in Hungary and Romania has at times removed an estimated 4 GW of baseload capacity from the broader regional system.
Cross-border congestion and evening pressure on markets
The simultaneous constraints highlight growing interdependence across Southeast European electricity markets. Lower generation in one country increasingly translates into stronger import demand, congested cross-border capacity, and higher prices elsewhere. Pressure is particularly visible during evening hours when solar generation ends but cooling demand remains elevated.
The summer of 2026 is becoming a stress test for individual generating assets and for the region’s ability to share limited flexible generation across increasingly interconnected markets. The pattern reflects how river-linked nuclear and hydro constraints propagate into system balancing needs.
