Dunkelflaute (prolonged low wind and solar output), operational consequences · Glossary

A Dunkelflaute is a weather situation in which wind and photovoltaic plants feed only little electricity into the grid over an extended period. The operational consequence describes what follows from this for day-to-day operation, dispatch planning and the safeguarding of the electricity system. It therefore does not simply mean that sun and wind are absent. The question is which other resources can replace the missing feed-in, when they are available, and whether they act at the right place in the grid. There is no generally binding definition with fixed thresholds for duration or feed-in share. The Bundesnetzagentur (Federal Network Agency) describes a Dunkelflaute as a persistent weather situation with little sunlight and little wind.

The key variable is the residual load. It is electricity demand minus the feed-in from weather-dependent renewable energy. If feed-in from wind and photovoltaics falls while demand stays the same or rises, the residual load grows. It must then be covered by dispatchable power plants, storage, flexible demand, imports or other reliable contributions. For operation, what matters is not only the amount of energy in kilowatt-hours or terawatt-hours, but also the required power in kilowatts, megawatts or gigawatts. A short Dunkelflaute may affect little energy in total but demand a great deal of power in individual evening hours. Conversely, a long phase with a moderate shortfall may require large amounts of energy from storage or fuels. An illustrative comparison: power corresponds to the diameter of a water tap, energy to the amount of water in the tank. For a Dunkelflaute, both are needed in sufficient measure.

Distinction from electricity shortage and supply gap

A Dunkelflaute does not automatically mean a power outage. Initially it only describes how much wind and solar power is generated. Whether this results in a supply problem depends on the available balancing options. This is shown by the Dunkelflaute episodes of 5 to 7 November and 11 to 12 December 2024 in Germany. Wholesale prices temporarily rose above 300 euros per megawatt-hour, peaking above 900 euros. The 2024 annual average was around 79 euros. According to the evaluation by the Bundesnetzagentur and the Bundeskartellamt (Federal Cartel Office), the dispatchable power plants were largely in operation. In the most expensive hours, according to the Bundesnetzagentur's estimate, about 4.5 and 3.4 gigawatts of market capacity respectively were still available, as well as roughly 12 to 13 gigawatts of reserves and balancing energy. Supply was secure at all times. The Dunkelflaute thus became visible mainly through the price, not as an interruption of supply.

One speaks of an electricity shortage only when demand cannot be fully met, or can be met only through interventions outside normal market operation. A supply gap, by contrast, is a planning statement: under certain assumptions, capacities or reserves are not sufficient. The Dunkelflaute itself is an actual or modelled weather and feed-in situation. In analyses of resource adequacy, Dunkelflaute episodes serve as stress cases. They show which resources must be available in unfavourable hours or over longer periods.

The term is often conflated with low annual yields from renewable energy. This leads to wrong conclusions. A year can have good average wind and solar yields and still contain several critical Dunkelflaute episodes. For operational security, what matters are the hours and days in which high demand coincides with low renewable feed-in. Annual balances obscure this temporal structure.

Duration, season and load profile

How heavily a Dunkelflaute strains operation depends above all on its duration. Individual hours with little wind and sun can be balanced by battery storage, pumped storage, short-term imports, flexible industrial loads or controllable power plants. If the situation lasts several days, the picture changes: storage levels fall, the availability of fuels and power plants becomes more important, and the market has to cover high residual loads over a longer time. In phases of one or more weeks, long-term storage, hydrogen-capable power plants, biogas, firm thermal capacity, load shifting and European balancing come into play above all.

The season also plays a role. Dunkelflaute episodes in winter are particularly demanding because photovoltaics then deliver little anyway owing to short days and a low sun, and electricity demand is often higher. The 2024 cases occurred in November and December. With increasing electrification of heat and transport, this relationship may intensify. Heat pumps raise electricity demand in cold weather unless they are operated flexibly through building mass, thermal storage or suitable tariffs. Electric vehicles create additional load, but their charging can be partly shifted in time.

A Dunkelflaute is therefore also a question of the load profile. Two days with the same amount of electricity can be very different operationally: for example, when demand peaks in the early evening or when industrial loads can be shifted. The most expensive hours in 2024 each fell between 5 and 6 p.m. In such situations, flexibility is worth only as much as it can concretely deliver: reducing consumption, increasing generation or providing stored energy precisely in the scarce hours.

Market, ancillary services and reserves

In normal operation, generators, storage and consumers respond first via the market. Day-ahead and intraday markets reflect expectations about demand, wind, sun, power plant availability and interconnector capacities. A Dunkelflaute can usually be foreseen a few days ahead. Prices then typically rise in the hours with high residual load. This makes flexible demand, storage discharge and dispatchable generation valuable. A price does not, however, create physical availability if plants are lacking, fuels are not on hand or grid bottlenecks limit transport.

For the transmission system operators, what counts is the physical balance in real time: generation and consumption must match at all times. The tools must be clearly distinguished here. Balancing energy compensates for short-term, unforeseen deviations, such as forecast errors or power plant outages. It is not intended to cover a foreseeable, multi-day deficit. Redispatch resolves grid bottlenecks by ramping power plants down in one region and up in another. It does not replace generation that is missing in the system as a whole. The Bundesnetzagentur therefore reports balancing energy and reserves as an additional safety buffer alongside market capacity.

In Germany, under the Energy Industry Act (Energiewirtschaftsgesetz, EnWG), there are, among others, two reserves with different purposes. Anyone discussing Dunkelflaute episodes should not confuse them:

Grid reserve under § 13d EnWG: The transmission system operators hold plants available, above all to manage grid bottlenecks, maintain voltage and secure a possible restoration of supply. The grid reserve thus has primarily a spatial, grid-related function.
Capacity reserve under § 13e EnWG: a strategic reserve outside the electricity markets. It compensates for power balance deficits when supply and demand on the electricity markets are not fully balanced. The plants feed in only at the request of the transmission system operators and may not sell their capacity on the market. The law provides for 2 gigawatts, subject to adjustment after regular review. A reduction in electricity consumption can also count as reserve capacity.

Reserve power plants are something different from plants that run regularly. A plant may run rarely and still be important for security of supply if it provides firm capacity in scarce hours. This gives rise to an economic problem: the energy market remunerates mainly energy generated and short-term scarcity. Holding rarely used capacity available, however, incurs its own fixed costs. This is the origin of the debates about capacity markets, strategic reserves, new hydrogen-capable plants, the power plant strategy (Kraftwerksstrategie) and the question of who bears the costs of this safeguarding. After the events of 2024, the Bundesnetzagentur considers statutory measures for the build-out of dispatchable capacity urgently necessary. It also sees a need for more flexibility on the supply and demand side, for example through a more flexible operating mode of biomass plants.

Typical misunderstandings

A widespread misunderstanding is that a high installed capacity of wind and photovoltaics replaces the same amount of firm capacity. Installed capacity describes only the maximum possible output in favourable weather. It says little about what is actually available during a wintry high-pressure situation with fog and no wind. For safeguarding, the reliable contribution in the critical hours counts, not the nameplate capacity on paper.

It is equally crude to treat imports across the board as either a secure or an unreliable solution. European electricity markets help to balance regional weather differences. Grid interconnections, different power plant fleets and different load profiles widen the balancing options. However, large-scale weather situations can affect several countries at the same time. Neighbouring countries then compete for similar resources, and free line capacity is not guaranteed. Imports are an important building block, but they do not replace safeguarding of one's own or safeguarding secured by contract.

Storage is also often classified imprecisely. Battery storage provides power very quickly and is valuable for price peaks and grid requirements. Its strength usually lies in the range of hours. A multi-day Dunkelflaute requires large amounts of energy or the possibility of recharging in between. Pumped storage, thermal storage, hydrogen storage and other long-term storage serve different tasks than home storage or grid batteries. Anyone who looks only at a storage unit's power overlooks its duration. Anyone who looks only at storage capacity overlooks the maximum output in critical hours.

It is also wrong to equate Dunkelflaute episodes with a failure of renewable energy. The expansion of weather-dependent generation rather changes the nature of the safeguarding required. In the past, an electricity system had to cope above all with power plant outages, fuel risks, load peaks and grid disturbances. With high shares of wind and photovoltaics, there is the added factor that their feed-in fluctuates in a correlated way over time. Planning does not become superfluous as a result, but must become more precise: resource adequacy, grid expansion, storage strategy, load management and European coordination must all refer to the same scarcity situations.

Costs and responsibilities

Safeguarding against Dunkelflaute episodes costs money. Costs arise not only for kilowatt-hours generated, but also for standby capacity, start-up capability, fuel logistics, grid interventions, reserve contracts, storage losses and the safeguarding of rare extreme hours. An electricity system with very low variable costs from wind and photovoltaics may nevertheless have considerable fixed costs for firm capacity. These costs do not disappear if they are absent from the wholesale price. They then appear elsewhere, for example in levies, capacity payments or government financing mechanisms. One example is the capacity reserve: its operators receive an annual remuneration, and the transmission system operators may claim the costs, less revenues, via the grid fees.

Responsibility is spread across several levels. Plant operators decide on investment and dispatch according to the market rules. Grid operators secure day-to-day operation and must manage bottlenecks and balancing deviations. Regulators and legislators determine which reserves are procured, which market incentives apply and how the costs are distributed. Municipalities, industry and households influence the demand side through heat pumps, charging infrastructure, self-consumption, storage and load management. Conflicts arise where technical possibilities, market rules and political responsibilities diverge.

Anyone who uses the term Dunkelflaute precisely makes visible which safeguarding is needed for high residual load. Which technology fulfils this task most cheaply is not answered by the term alone. For that, duration, frequency, simultaneity with neighbouring countries, grid restrictions, storage durations, fuel availability and demand flexibility must be considered together. The operational consequence is to provide power and energy at the right time and in the right place. Average annual quantities are not sufficient as a yardstick for this.

Sources

Bundesnetzagentur und Bundeskartellamt veröffentlichen Untersuchung zu Strompreisspitzen während Dunkelflauten 2024 — Bundesnetzagentur; 2025-10-21. Pressemitteilung, Abschnitte Einleitung, Ergebnisse der Bundesnetzagentur, Hintergrund
§ 13e EnWG Kapazitätsreserve — Bundesministerium der Justiz / Bundesamt für Justiz (gesetze-im-internet.de); unbekannt (abgerufen 2026-11-11). § 13e Abs. 1 bis 5 EnWG
§ 13d EnWG Netzreserve — Bundesministerium der Justiz / Bundesamt für Justiz (gesetze-im-internet.de); unbekannt (abgerufen 2026-11-11). § 13d Abs. 1 EnWG (laut Suchergebnis-Auszug)

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Edited by eCMS on 11.11.2026 19:42