Dark Doldrums
Key Points
Dark doldrums definition
Dark doldrums are periods when wind and solar power generation drops to exceptionally low levels (<10% of installed capacity). Since wind and solar together now produce nearly half of Germany’s electricity, these phases are an increasing challenge for the power system and for ensuring security of supply.
Our analysis clearly shows that periods of low wind are not a marginal phenomenon, but rather an inherent feature of a weather-dependent energy system.
Occurrence
How often do dark doldrums occur?
Dark doldrums are neither rare nor fringe events—they happen regularly.
1,435 dark doldrums
A total of incidents lasting more than ten hours have occurred over the last ten years (2016–2025).
144 dark doldrums
The average amount of incidents, per year.
More often than every 3 days
On average, therefore, periods of low activity occurred.
Duration
How long do dark doldrums last?
Our analysis of dark doldrums lasting at least ten hours shows that, between 2016 and 2025, these events averaged 12.9 hours—so typically more than half a day. The electricity system must also be ready for much longer dark doldrums. While less frequent, they still happen regularly: a 24-hour dark doldrum occurs nearly every month, three-day doldrums around twice a year. Extreme dark doldrums of more than five days are exceptional, but the security supply must be guaranteed even in these cases. On average, such extended events happen every three and a half years. In 2023, Germany experienced a nearly week-long dark doldrum (161 hours).
Security of supply
How can the security of supply be ensured during dark doldrums?
If demand outpaces the amount of power generated from renewable sources, power stations, imports, demand flexibility and storage facilities are brought into play. Battery storage is playing an increasingly vital role in this regard: it absorbs surplus energy from renewable sources and releases it during periods of low generation, thereby helping to mitigate price spikes. They can also bridge short periods of low renewable generation. That is why Uniper is also investing in the expansion of storage infrastructure.
However, the storage capacity of battery storage systems is not sufficient to bridge prolonged periods of low wind and low sunlight lasting ten hours. Power stations that can generate electricity reliably and flexibly, regardless of weather conditions, therefore remain a key component of security of supply.
Power plant strategy
What does this mean for the German electricity system?
Due to the ongoing phase-out of coal-fired power generation, Germany faces the threat of a capacity gap in dispatchable power. To ensure that sufficient dispatchable power remains available in the future, the federal government has announced that it will issue tenders for long-term capacity as part of the Power Plant Strategy (StromVKG). These capacities must be able to feed electricity into the grid at full load for at least ten hours. Power plants meet this criterion because they can generate electricity reliably at any time without being restricted by weather conditions or charging cycles. They are therefore also suitable for bridging longer periods of low wind and low sunlight (>10 hours), which are a structural component of the German power system.
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