The summer of 2026 has highlighted just how closely water and electricity are connected.
Across drought- and heat-affected parts of Europe, low river levels, reduced flows and unusually warm water have constrained the operation of some nuclear and thermal power plants. At the same time, persistent heat has increased electricity demand for cooling across homes, offices, commercial buildings and data centres.
The result is a difficult combination for power systems: generation can become more constrained just as weather-driven demand is rising.
Romania and Poland have provided two of the clearest examples this summer.
Romania: when the Danube falls too low
Romania’s entire nuclear generation comes from a single site: the Cernavodă nuclear power plant on the Danube.
Its two operating units have an installed capacity of approximately 700 MW each and, under normal conditions, together provide around one fifth of the country’s electricity.
Both depend on the Danube for essential plant and cooling-water requirements.
On 28 July 2026, operator Nuclearelectrica carried out a controlled shutdown of Unit 1 as the Danube fell to exceptionally low levels during a period of severe hydrological drought.
Authorities and plant operators subsequently took measures intended to improve water availability at the site, including efforts to redirect more flow towards the plant. These measures produced some improvement, but not enough to remove the operational risk.
As river levels continued to fall, Nuclearelectrica began a controlled shutdown of Unit 2 on 13 August, leaving both reactors offline.
The impact had already extended into the commercial side of the business. On 10 August, before Unit 2 was shut down, Nuclearelectrica obtained force-majeure certificates relating to electricity-supply obligations affected by the drought and the shutdown of Unit 1.
The episode demonstrates an increasingly important risk for thermal generation: the availability of fuel alone does not determine whether a plant can operate. Water availability and temperature can become operational constraints in their own right.
Poland: low Vistula flows constrain coal generation
Poland faced a similar physical challenge, this time affecting coal-fired generation.
On 4 August, utility Enea reduced output at units at the Kozienice and Połaniec power stations as low water levels on the Vistula restricted cooling-water availability.
The reductions removed approximately 1.3 GW of generating capacity from the system — around 2% of Poland’s total installed capacity.
Near Kozienice, Vistula flow fell to around 135 m³/s on 5 August. The power station relies on a water-intensive open-cycle cooling system, making low river flow a direct operational constraint.
As reserve margins tightened, Poland’s transmission system operator PSE declared two capacity-market system stress periods between 17:00 and 19:00 on 4 August.
The episode did not represent a failure of coal supply. It was a reminder that large thermal power stations depend on environmental conditions beyond the fuel arriving at the plant.
France: a different mix of environmental constraints
France also experienced substantial nuclear unavailability during the August heat.
On 12 August, around 20% of the country’s nuclear generating capacity was unavailable or operating at reduced output for a combination of environmental and operational reasons.
The causes were not all the same.
At several inland plants, high river temperatures and environmental limits on cooling-water discharge affected operations. At Gravelines on the northern coast, however, multiple reactors were taken offline after a large jellyfish swarm disrupted cooling-water intake systems.
The French example is therefore different from the low-flow events in Romania and Poland, but it illustrates the same broader principle: the output available from large conventional power stations can be influenced by environmental conditions that are difficult to separate from weather and climate.
The other side of the equation: heat drives demand
Generation constraints are only one side of the problem.
Heatwaves can also sharply increase electricity demand as cooling loads rise across residential, commercial and digital infrastructure.
The same persistent weather pattern that produces high temperatures can also coincide with low river flows, warmer cooling water, reduced wind generation and other stresses on the power system.
That creates a two-sided squeeze: weather can increase demand while simultaneously limiting parts of supply.
Across European power markets this summer, heat-driven demand coincided at times with lower wind output, thermal and nuclear constraints, and elevated fuel costs. Together, these factors contributed to sharp movements in electricity prices, with several markets reaching multi-year highs during periods of extreme heat.
In parts of southeast Europe, the strain also led authorities and system operators to introduce emergency conservation and demand-management measures.
No single weather variable explains these market movements. Electricity prices emerge from the interaction of demand, generation availability, fuel costs, renewable output, interconnector capacity, storage and network constraints.
But weather influences several of those variables at the same time.
For energy traders, utilities and grid operators, the challenge is therefore broader than monitoring the river level beside an individual power station.
It is understanding how weather is changing the balance of the system, and how quickly.
Turning weather-driven demand into a signal
This is where the OpenWeather Energy Demand Index, part of the Climate Indices suite, can add another layer of information.
The index uses Heating Degree Days (HDD) and Cooling Degree Days (CDD) to quantify the weather-driven component of changing energy demand.
During a heatwave, rising Cooling Degree Days provide an indication of increasing cooling requirements. During cold periods, Heating Degree Days provide the corresponding signal for heating demand.
This does not replace a full electricity-load forecast. Actual demand also depends on factors such as industrial activity, working patterns, energy prices, holidays and consumer behaviour.
But it provides traders, utilities and grid operators with an early indicator of weather-driven demand pressure that can be incorporated into load-balancing, capacity-planning and market-analysis processes.
That becomes particularly useful when the generation side of the system is already constrained.
Looking at both sides of the weather equation
The OpenWeather Climate Indices suite also includes a Solar Radiation Index, providing a complementary view of weather-driven solar-generation potential.
That matters during summer heatwaves because strong daytime solar output can offset part of the increase in cooling demand, even while conventional generation is facing other constraints.
For operators and traders, the useful question is therefore not simply “how hot will it be”?
It is - what will that weather pattern do to demand, renewable generation and the wider balance of the power system?
Looking at weather through that operational lens allows energy businesses to move beyond individual forecast variables and towards a clearer understanding of their potential market impact.
Preparing for the next low-water summer
The conditions seen across Europe in 2026 are unlikely to remain exceptional.
European climate assessments project that heatwaves and droughts will become more frequent and intense across much of the continent as the climate warms.
That does not mean every hot summer will produce nuclear or thermal generation cuts, nor that every drought will lead to power-system stress.
But it does increase the value of understanding how weather affects both sides of the electricity balance.
Power plants beside rivers cannot always be made completely independent of hydrological conditions. Weather-sensitive demand cannot be eliminated either.
What energy businesses can improve is their visibility.
By translating forecasts into indicators of weather-driven demand and renewable-resource availability, weather intelligence can give traders, utilities and grid operators another layer of information for making decisions when heat, drought and tight power-system conditions converge.