Cold Dunkelflauten in Germany: a hidden challenge for the energy transition

The topic of cold Dunkelflauten is increasingly sparking heated debates: some use this phenomenon to highlight how vulnerable our energy supply could be in light of the energy transition. Others respond with a range of proposed solutions to enhance flexibility in the power system.

In recent years, the media has increasingly focused on this phenomenon, often with dramatic headlines. But how often do such situations actually occur? And what are the real impacts on power supply and prices? This is precisely where our analysis begins.

We examined the frequency and impact of such events between 2015 and 2024 to assess whether media coverage is backed by solid data.

What is a cold Dunkelflaute?

A cold Dunkelflaute refers to a weather situation where there is little to no wind or solar power generation for several hours or even days. At the same time, electricity demand rises sharply due to low temperatures. This results in a significant increase in what is known as the residual load – the amount of power demand not covered by variable renewable energy sources.

For the purposes of our analysis, we define a cold Dunkelflaute as a period during which the residual load exceeds 60 GW for several consecutive hours.

Methodology and data analysis

To quantify the extent of the issue, we analysed comprehensive datasets from the past ten years. The following parameters were considered:

• Power generation from onshore and offshore wind, as well as solar energy

• Grid load (electricity demand)

• Residual load (defined as total load minus renewable energy input)

• Day-ahead electricity prices

To qualify as a Dunkelflaute, a time-based criterion must also be met. The longer the residual load exceeds the defined threshold, the greater the challenge for ensuring supply security.

We categorised three types of events:

• 6h Event: residual load exceeds 60 GW for at least 6 consecutive hours

• 12h Event: residual load exceeds 60 GW for at least 12 consecutive hours

• 2T Event: two consecutive days with either a 6h or 12h event

We also examined how the number of 15-minute intervals with residual load over 55 GW has developed over the years and whether there is any correlation with electricity prices.

Evaluation

The number of 15-minute intervals with high residual load (>55 GW) has generally declined over the past 10 years. Extreme values over 70 GW are now rare. A particularly notable trend is the continuous decrease since 2015, possibly indicating a shift in the generation mix and improvements in grid management.

In addition to frequency, the duration of high residual load phases is crucial. How often do such phases last several hours or even days? Our analysis shows that the number of 6h and 12h events was relatively high until 2017 but declined significantly afterward. Since 2018, these events have become rarer and have stabilised at a lower level. Particularly long Dunkelflauten lasting more than 12 hours are now less frequent than in the previous decade.

Day-ahead prices: the impact of Dunkelflauten

Trends in residual load and the frequency of extended Dunkelflauten also affect wholesale electricity prices. During times of high residual load, day-ahead prices tend to rise as conventional power plants must step in or electricity must be imported to make up for the shortfall in renewable generation.

The following diagram compares three different base prices:

1. The average annual base price

2. The base price on days with Dunkelflauten (specifically 6h events)

3. The base price on all days without Dunkelflauten within the same months in which Dunkelflauten occurred

This differentiation allows us to better understand the impact of cold Dunkelflauten on price developments.

Our analysis shows that the price differences between days with and without Dunkelflauten were minor until 2020. Starting in 2021 – particularly in 2022 – these differences increased significantly (see Figure 4).

One major factor was the energy price crisis of 2021/2022, driven by high gas prices and geopolitical uncertainty. Gas and oil-fired plants, which often set the price during Dunkelflauten, operated at higher costs, pushing electricity prices upward. In addition, rising CO₂ prices further increased the cost of conventional generation.

The European electricity market also played a role: in 2022, many French nuclear power plants were offline, increasing demand for German electricity. In 2023, gas and electricity prices declined again, and the price gap between days with and without Dunkelflauten narrowed. However, in 2024, this gap widened once more: while the average annual base price fell compared to the previous year, average prices on days with Dunkelflauten rose significantly.

Conclusion

The cold Dunkelflaute phenomenon is occurring less frequently, but when it does occur, it is expensive compared to the usual market electricity price. Cold Dunkelflauten influence electricity prices only temporarily and are not the main drivers of long-term price developments. Geopolitical events, fuel costs, and European market dynamics have had a much larger impact on electricity prices in recent years.