Clean energy sources in global power generation are on track to break new records over the 2025-2027 forecast period. Low-emission sources – renewables and nuclear – are expected to meet all global demand growth out to 2027. Solar PV is set to become the second largest low-emissions source of electricity generation in the world by 2027, after hydropower. Renewables, collectively, will surpass coal-fired generation in 2025 and coal’s share will decline below 33% for the first time in the last 100 years. Nuclear generation will reach a new record high in 2025, driven by a recovery in output in France and Japan, and new reactors entering operation in China, India and Korea, among other countries. Nuclear energy will continue to set a new record every year thereafter. The share of low-emissions sources is forecast to increase from 41% in 2024 to 47% in 2027.

As the share of renewable energy sources in the electricity generation mix rises, understanding periods with reduced wind and solar PV generation due to weather conditions becomes important. While such events can potentially strain the power system, having enough dispatchable capacity and long-duration storage will be essential. 

Temporary periods with reduced wind and solar PV generation may put additional strain on the power system, especially if they occur during periods of high electricity demand, such as during colder winter seasons with increased heating demand, or hotter summers with higher cooling load. During these periods, power demand is met predominantly by dispatchable power plants and using various flexibility measures. These events can also lead to temporary and briefly higher prices on the wholesale markets if supply is tight. Having sufficient (low-emissions) dispatchable capacity and long-duration storage, among other flexibility options such as demand-side flexibility and interconnections, is important to effectively manage such periods.

In November and December 2024, there were several occasions in Northern Europe when combined wind and solar PV electricity generation was very low, the so-called compounded VRE droughts or Dunkelflaute events, which led to tighter supply and several hours of extremely high electricity prices on the wholesale markets. In 5-7 November and 11-12 December Germany and the surrounding regions were affected. In early 2025, on 8 January, the United Kingdom had a relatively localised Dunkelflaute that lasted for about a day. Low wind availability during the nighttime combined with interconnector unavailabilities, power plant maintenance outages and elevated electricity demand led the national energy system operator (NESO) to issue a notice due to low system margins.

All these events were managed successfully without any impact on the supply of electricity, showing the resilience of the power systems and the underlying market mechanisms. The price spikes observed during the period of a few hours only had a very limited impact on average prices but acted as important signals to incentivise flexible generators to produce more and for flexible consumers to reduce their consumption, while also facilitating the efficient import and export of electricity. 

As the share of renewable energy sources in the electricity generation mix rises in many regions and they enter higher phases of VRE integration, understanding VRE droughts better and preparing for them accordingly becomes more important. If the VRE drought lasts only 1-2 days, short-term storage capacities and demand-response can help with smoothing the residual load. However, if the situation persists for longer durations, it becomes increasingly difficult to recharge storage capacities and utilise short-term flexibility options. Enhanced grid interconnection across different geographies can help balance the overall energy supply to an extent, given the locational price signals are reflective of the system costs during such events. However, as meteorological conditions can affect the wind and solar output for multiple neighbouring power systems, expanding interconnection capacity may not be enough on its own.

Consequently, having sufficient dispatchable capacities and long-duration storage becomes important to effectively manage longer-lasting VRE droughts. In the short-term, pumped hydro storage is a proven and mature technology for longer duration storage that can provide flexibility over a timescale of days to weeks, with thermal energy storage and hydrogen storage being relevant low-carbon technologies in the longer term where the timescale of flexibility can be extended to months (seasonal flexibility).

As highlighted in the IEA report Managing the Seasonal Variability of Electricity Demand and Supply, hydro and dispatchable thermal power plants will remain important providers of secure capacity, alongside other sources of system flexibility discussed above. In a low-emissions power system, these thermal capacities also need to be correspondingly based on low-emissions fuels or include carbon capture and storage. Ensuring sufficient dispatchable capacity in the long-term may require mechanisms and market designs that properly value these generators' critical services, even if they operate infrequently over the course of a year. When conducting resource adequacy assessments, it is crucial to consider the unpredictable nature of weather impacts, which can influence the planning of investments, potential generator retirements, and policy decisions.