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Hitzeflaute (eng. Heat Lull)

The effects of climate change are becoming increasingly noticeable in everyday life: temperatures are rising, and summers in particular are getting hotter.


This is especially apparent in average daily temperatures during the summer months, which, according to the German Environment Agency, continue to rise. Across Germany, this summer is averaging around 19.3 degrees Celsius (°C), just four tenths of a degree below the previous record of 19.7 °C set in 2003.


However, the growing frequency of hot, sunny weather does not only affect people, their health and the environment. It is also changing the dynamics of the electricity market, with so-called “Hitzeflauten” occurring more frequently.


What is a Hitzeflaute?

A Hitzeflaute (eng. heat lull) is an electricity-market phenomenon that occurs when high temperatures coincide with low wind speeds. High electricity demand combined with low wind generation puts pressure on the market and drives up prices.


A typical heat-lull day is also particularly sunny: PV systems therefore feed large amounts of electricity into the grid between 6 a.m. and 8 p.m., keeping power prices relatively low during the day.

At the same time, prolonged periods of heat and drought are usually associated with persistent high-pressure systems. These limit air circulation, so there is very little wind and wind turbines generate correspondingly little electricity.

Together, these two factors make the evening hours critical: once solar generation falls towards zero, while wind remains scarce and electricity demand stays high, conventional power plants have to step in. Gas- and coal-fired plants then set the market-clearing price, causing electricity prices to rise.


Week 26 provides a good illustration of this phenomenon. According to Energy Charts, solar generation reached extremely high levels that week, while wind generation was negligible. The figure below shows day-ahead price movements on 24 June 2026.


Graphs showing Day-ahead price development on 24 June 2026
Fig.: Day-Ahead price development (24 June 2026)

The graph follows the typical pattern of a heat lull. Prices fall around sunrise, shortly before 7 a.m., and reach their low at 12:45 p.m., at €64/MWh. They remain relatively low throughout the day before rising sharply as the sun goes down and solar generation declines. Gas- and coal-fired plants then step in and set the price, pushing electricity prices to a daily high of EUR 747/MWh at 8:45 p.m.


For comparison, the average day-ahead price in June was EUR 110/MWh. The peak price on this day was therefore 579% higher.


What happens in the market on these days?

On hot days, electricity demand rises noticeably, for several reasons.


Energy-intensive cooling systems in industry run at full capacity. Production and logistics processes in particular require significantly more cooling at high outdoor temperatures, for example to keep machinery, server rooms and cold chains operating reliably. Air conditioning is also becoming more common in private households. Overall, this puts greater strain on the electricity system.


Heat also affects the supply side. Conventional power plants operate less efficiently under these conditions and, in some cases, their available capacity is restricted. A recent example is the French energy company EDF, which temporarily shut down several nuclear reactors in summer 2026 and had to reduce output at other plants because persistent heatwaves raised river temperatures. This was due to statutory environmental requirements: when river temperatures are already elevated during a heatwave, operators must reduce output or shut plants down to comply with limits on discharge temperatures. In such conditions, power plants can no longer discharge heated cooling water into already warm rivers without restriction.


In short, when generation capacity is lost in this way while demand remains high or even increases, the impact is immediately reflected in prices.

 

Fazit

The current price spreads in the market show that the problem is not a lack of electricity, but a lack of flexibility. During the day, so much solar power is generated that PV systems sometimes have to be curtailed because the grid or the market cannot fully absorb the electricity. That energy is then no longer available in the evening, precisely when it is needed. Expensive conventional power plants therefore have to step in to close the gap.


Battery storage systems offer a reasonable solution: they can absorb surplus solar power during the day and make it available later, when it is needed most.

The heat-lull phenomenon therefore shows that the need for storage capacity is both real and recurring – and that battery storage is one of the most effective solutions already available today.


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