f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Dunkelflaute (prolonged low wind and solar output)

power system security of supply renewable energies residual load flexibility

A Dunkelflaute is a weather situation in which wind power and photovoltaic plants simultaneously generate little electricity for several hours up to several days. The term does not mean a power outage. It describes a phase in which the two most important weather-dependent renewable energy sources feed in hardly anything. Dunkelflauten matter most in late autumn and winter. Days are short then and the sun stands low. In addition, large-scale high-pressure situations often bring little wind, fog or high fog. In meteorology, the large-scale weather pattern "High over Central Europe" plays the main role here.

Technically, a Dunkelflaute concerns both power and energy. Because wind and sun deliver little, the residual load, that is, electricity demand minus weather-dependent generation, rises at every single moment. If this state persists, a second question arises: what quantity of energy must be covered by other sources over the entire period? A single low-wind night therefore places different demands on the system than a multi-day winter period with little sun and weak wind across large parts of Europe.

No uniform definition: thresholds and duration

There is no statutory or generally binding definition of a Dunkelflaute. Studies and authorities work with their own thresholds. Almost always, the combined generation of wind and photovoltaics is measured as a share of installed capacity or rated capacity. A minimum duration is added to this. Two examples from Germany show how different these definitions are:

  • In 2018, the German Meteorological Service (Deutscher Wetterdienst, DWD) evaluated, by way of example, how often the average generation from wind and sun in an area stayed below 10 percent of rated capacity for 48 hours. For this, it assumed the performance characteristics of the plants at that time. According to reports on the study, this case occurred in Germany on average about twice a year when onshore wind, offshore wind and photovoltaics are considered together. For Europe as a whole, the evaluation arrived at about 0.2 cases per year.

  • In its 2024 investigation of price peaks, the Federal Network Agency (Bundesnetzagentur) applied a different threshold: wind and PV plants deliver less than 15 percent of their installed capacity for at least 48 hours without interruption.

How often and how long Dunkelflauten occur therefore depends strongly on the definition. Stricter thresholds yield rarer and shorter events. Larger-scale analyses generally yield fewer cases than looking at a single country. Figures from different sources can therefore only be compared if the threshold, minimum duration, region, plant mix and data basis are known. Expansion also changes the statistics: a mix with more offshore wind and photovoltaics reacts differently to the same weather situation than a purely onshore wind fleet.

Delimitation: not a blackout and not a shortfall in annual generation

A Dunkelflaute is not the same as a blackout. A blackout is an unplanned large-scale power outage. It can occur when grid operation, the power balance or protection systems fail. A Dunkelflaute, by contrast, is a state of electricity generation that usually becomes apparent in weather forecasts a few days in advance. It places higher demands on the power system but does not automatically lead to an interruption of supply. Whether it becomes a problem depends on dispatchable power plants, storage, load management, imports, grid capacities and the rules of the electricity market. The Dunkelflauten in Germany in November and December 2024 show this: according to the Federal Network Agency, supply was secured by reserves at all times.

A Dunkelflaute also does not simply mean "too little renewable energy". Wind and solar plants can generate a great deal of electricity over a year and still contribute little in certain hours. Annual energy output and firm capacity are different quantities. A photovoltaic plant can deliver high yields in summer and contribute nothing to covering load on a cold winter night. A wind fleet can generate a lot of electricity on annual average and still remain far below its rated capacity during large-scale high-pressure situations. Anyone who mixes up installed capacity, annual generation and available capacity in critical hours describes the challenge imprecisely.

Why Dunkelflauten matter for the power system

In the power system, generation and consumption must be in balance at all times. The grid itself stores very little energy. If electricity from wind and sun is missing, it must therefore be replaced at the same moment, or consumption must adjust. The more weather-dependent generation there is, the more the central planning question shifts. It is not enough to generate enough electricity over the year. The system must also be able to bridge hours and days of high residual load reliably.

Very different means are available for this. Dispatchable power plants supply electricity independently of wind and sun, as long as fuel is available and the plants are ready for operation. Storage shifts electricity from times of high feed-in to times of scarce generation. Short-term storage fulfils different tasks than seasonal storage. Flexible consumers can shift loads, for example in industrial processes, heat storage, electrolysers or the charging of electric vehicles. Imports help if neighbouring countries have surpluses or free dispatchable capacity at the same time. Each of these options has technical limits, costs, lead times and institutional prerequisites.

Dunkelflauten therefore do not concern generation alone. They also touch on grid planning, market design, security of supply and the organisation of flexibility. Anyone who looks only at the average levelised costs of electricity of individual technologies overlooks the costs of capacity in rare but critical hours. Conversely, anyone who looks at individual extreme hours without the annual context easily overestimates the need for reserve capacity. System planning must bring both together: generating large quantities of electricity cheaply and reliably covering high residual load.

Example: the Dunkelflauten in winter 2024

At the beginning of November 2024, there was only little wind in Germany for a while. The lull was particularly pronounced on 6 November, when solar irradiation was also below average. A second Dunkelflaute followed on 11 and 12 December 2024. In both phases, wholesale prices temporarily rose to more than 300 euros per megawatt hour. The peak value was 936 euros per megawatt hour in day-ahead trading for the hour from 5 to 6 p.m. on 12 December. By comparison, a megawatt hour cost around 79 euros on average in 2024.

In their investigation, the Federal Network Agency and the Federal Cartel Office (Bundeskartellamt) found no indications of market abuse. The lignite and hard coal plants reported as available largely ran during the expensive hours. Gas-fired and pumped-storage plants still had remaining capacity. According to an estimate by the Federal Network Agency, about 4.5 and 3.4 gigawatts of market capacity respectively were still available in the most expensive hours, plus about 12 to 13 gigawatts of reserves and balancing energy. The example shows the typical effect of a Dunkelflaute. It is first of all a scarcity signal in the market with high prices, not a supply failure. The authority nevertheless sees an urgent need for new dispatchable capacity and more flexibility on the supply and demand sides.

From a meteorological perspective, the DWD classified the November lull as not unusual. For the large-scale weather pattern "High over Central Europe", the long measurement series of the winter half-years show no significant trend, neither in the number of days nor in the longest continuous duration. The period examined thus does not indicate that climate change has so far increased the risk for wind power generation.

Duration, spatial extent and simultaneity

Not every low-wind hour is a Dunkelflaute. The term only becomes meaningful when duration and spatial extent are taken into account. A local drop in wind can be balanced by other regions if there is wind or sun there and the grid has enough transmission capacity. A large-scale Dunkelflaute across several countries removes these balancing options. Requirements for dispatchable capacity and stored energy then increase.

European interconnection reduces the risk, because the weather is not the same everywhere and electricity can be traded across borders. The DWD evaluation from 2018 also shows this. Interconnection does not solve the problem completely, however. Winter high-pressure areas can affect large parts of Central and Northern Europe at the same time. A country can also only import if its neighbours can export at that very moment and the lines are not fully loaded. Import capability is therefore not a fixed figure. It depends on the available capacity abroad, grid capacity, market rules and simultaneous demand.

A special form is the cold Dunkelflaute. Here, low wind and solar generation coincides with low temperatures. Electricity demand can then rise, for example because heat pumps have to supply more heat and operate with lower efficiency in severe cold. With the electrification of heat, transport and industry, this coupling is becoming more important. More electricity consumption is not a problem in itself if load profiles, storage and controllability are planned in. If such provisions are missing, additional load in unfavourable hours increases the residual load.

Storage solves part of the task, but not every task equally

In debates about Dunkelflauten, storage is often treated as a single category. This loses the differences in power, storage duration, efficiency and cost. Battery storage is particularly suited to short-term balancing, frequency support, shifting solar power within a day and smoothing peaks. If it is to bridge a multi-day Dunkelflaute on its own, it must hold very large amounts of energy. This is expensive, because storage capacity for rare events must be financed. A calculation example shows the order of magnitude: if 20 gigawatts of residual load must additionally be covered for three days, that already amounts to around 1,440 gigawatt hours.

Long-term storage can bridge longer periods but has other limits. This includes hydrogen, synthetic fuels, large heat storage and hydropower storage with large reservoirs. Many pumped-storage power plants are designed more for several hours than for days. Hydrogen must be produced, stored, transported and, when needed, converted back into electricity or used directly. A considerable part of the energy is lost in the process, and the infrastructure must be built beforehand. Such storage is therefore not a cheap all-purpose solution. It is one building block for cases in which other flexibility options are not sufficient or become too expensive.

The economic question is therefore not which technology could theoretically bridge a Dunkelflaute on its own. What matters is the cheapest combination of generation surpluses, grid expansion, flexible consumption, short- and long-term storage and firm capacity. The options compete not only technically but also through rules: who may control, who is remunerated, which risks do suppliers, grid operators or consumers bear, and which capacities are held available for rare scarcity phases?

Misunderstandings in politics and the media

A common misunderstanding is to take Dunkelflauten as proof that a high share of renewable energy is fundamentally not viable. This does not follow from the term. Dunkelflauten only show that weather-dependent generation needs complements. They do not say which complements are technically suitable and economically sensible. A power system with a lot of wind and solar can be operated very securely if the remaining residual load is covered by plannable resources, flexible demand and sufficient grids.

The opposite misunderstanding is to dismiss Dunkelflauten as a marginal problem because they are rare. Rarity does not make them unimportant for security of supply. Power systems are designed precisely for rare stresses, because missing capacity can cause high consequential costs within just a few hours. Planning must therefore not rely on average values. It must examine which firm capacity is really available in critical weather years and how several unfavourable factors can overlap. These include cold, power plant outages and scarce imports.

"Fully renewable supply" also becomes misleading if the time dimension is left open. A region can, on an annual balance, generate as much renewable electricity as it consumes and still depend on imports, storage or dispatchable plants during Dunkelflauten. The annual balance and hourly security of supply answer different questions. Both are legitimate but must not be mixed up.

Institutional significance

Dunkelflauten raise the question of how security of supply is organised and paid for. In an electricity market with a lot of variable generation, scarcity prices arise mainly in hours of high residual load. Prices then rise because flexible demand, storage or power plants are scarce. The price peaks of winter 2024 are an example of this. Such signals can trigger investment if market participants trust that scarcity prices will be accepted politically and will refinance investments. If these revenues are regarded as uncertain, capacity mechanisms, strategic reserves or other state-regulated safeguards are demanded. In Germany, the Federal Network Agency considers statutory measures for the build-out of dispatchable capacity urgently necessary. The Federal Cartel Office wants to use the upcoming tenders also to reduce the high market concentration in electricity generation.

The Dunkelflaute thus also becomes a question of governance. It requires clarity about which role the market assumes, which reserves the state organises, which obligations suppliers bear and how grid operators prepare interventions. Technical feasibility is not enough if responsibilities remain unclear or flexible plants earn no reliable revenues. Conversely, poorly designed safeguards cause costs that then show up in grid charges, levies or taxes.

Anyone who uses the term precisely gets beyond blanket statements such as "too little wind" or "too many renewables" in the debate. A Dunkelflaute is a concrete combination of low weather-dependent feed-in, increased residual load and a minimum duration, whose thresholds differ depending on the source. It does not automatically explain a power outage and does not replace analysis of the grid, market, storage and firm capacity. Its value lies in making visible the hours in which a renewable power system must show that it can balance bottlenecks.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:42

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.