Dark doldrums - a
A threat to the energy supply or a driver for the energy transition?

During a dark doldrums, the proportion of electricity generation from wind and solar power plants in the electricity mix is massively reduced. This can be the case on relatively dark winter days with little wind in particular. The lack of electricity generation must then be compensated for by conventional power plants and increasingly also by battery storage systems in order to cover the demand for electricity and ensure grid stability.

The share of installed wind power capacity (onshore and offshore) amounted to around 73 GW at the beginning of 2025 and therefore around 27 per cent of the installed capacity in Germany. At 100 GW, the share of installed solar power capacity was around 37 per cent of installed capacity.^[1] A loss of generation capacity from wind and sun on dark and windless days means a reduction of around 64 per cent of the total installed capacity. This is an enormous proportion, which often raises the question among the public as to whether there is still sufficient security of supply.

Consideration from a capacity and price perspective

In our experience, questions or even worries are not justified: The peak load - i.e. the highest demand for electricity - was 75 GW in Germany. This load is currently matched by installed capacities of over 90 GW - excluding wind and solar plants. For this purpose, we deliberately only consider the capacities that are freely available on the market - we do not take into account power plants that have been marketed as reserves and are no longer available on the free electricity market.

Wholesale pricing is determined by supply and demand: if supply becomes scarce, the price rises. This price signal determines how demand is met: Is it cheaper to ramp up (reserve) power plants or import electricity from neighbouring countries?

Option 1: On days with a dark doldrums, expensive power plants move into the merit order and determine the price. This can easily amount to several thousand euros per megawatt hour for individual hours on the electricity spot market.

Option 2: If there is availability in neighbouring countries and prices are low, the electricity flows from the low-price area to the high-price area. In 2024, the maximum physical electricity flows from other countries to Germany were around 15 GW. ^[2]

Security of supply is therefore currently assured with the existing power plant capacities and the possible marginal flows.

Rising number of negative prices

Important fact: While renewable energies lead to higher prices on some days, there are also days when a lot of sun and wind with low demand lead to low or even negative prices. In 2024, there were a total of 459 hours with negative prices in Germany. The trend is pointing steeply upwards with the increasing expansion of renewables: in 2023 there were around 300 hours with negative prices, in 2022 there were only around 70 hours.

Conclusion

The dark doldrums are characterised by days with little renewable generation and high consumption and are sometimes used to cast doubt on the energy transition and security of supply. However, the installed capacity in Germany, cross-border trading and the hours with negative prices indicate that the electricity market will function even with the expansion of renewables.

In addition, the hourly volatility on the spot market will be reduced in future by the expansion of storage solutions, so that the price extremes - higher prices during dark doldrums and low or even negative prices for solar and wind - will be mitigated. Ultimately, it is precisely these price signals that show that the market is working: they control imports and exports and give assets such as battery storage and flexibility a value that enables investment.

Do you have questions about the dark doldrums or price signals? Please get in touch with us!

<a class="arrow">renewables@vattenfall.de</a>

<a class="button">To the Article Overview</a>
 

^[1] BNetzA: Statistiken ausgewählter erneuerbarer Energieträger zur Stromerzeugung - November 2024.

^[2] Spitzenlast und grenzüberschreitende Stromhandel, s. Energy-Charts https://www.energy-charts.info/charts/power/chart.htm?l=de&c=DE.