But while solar panels feed on sunlight, Europe is in the grips of a record-breaking heat wave, and extreme heat is no friend to solar energy producers.

The heat that has been scorching parts of the U.K. and western and southern Europe has set temperature records, started wildfires, damaged infrastructure, and is leaving behind a mounting death toll.

High temperatures have sent electricity demand in Europe soaring, and combined with an ongoing shortage of natural gas on the continent since Russia’s invasion of Ukraine, renewable energy sources such as solar have had to step up.

Over the weekend, Germany broke the country’s record for solar power output, with even higher levels of electricity generation expected this week as the heat wave rages. 

But if temperatures remain elevated for long, it might actually risk slowing down solar energy’s output.

Solar panels around the world are manufactured and tested to function optimally at around 77°F, or 25°C, and the range in which most solar panels can still operate at peak efficiency is between 15°C and 35°C. Any hotter, and utility and installation companies warn that a panel’s efficiency can start dropping fast.

How they work

Solar panels absorb photons to energize electrons, causing them to flow and create electricity.

But when a panel heats up too much, its electrons are already in an excited state once sunlight is absorbed, reducing efficiency and the voltage it can generate.

Depending on where a solar panel is installed, high temperatures can reduce its electricity output efficiency by anywhere from 10% to 25%, according to CED Greentech, a leading solar panel equipment supplier in the U.S. 

And with temperatures in Spain, Britain, and other European cities already surpassing 40°C or expected to hit that mark soon, solar energy’s output could soon take a hit, and Germany’s record-breaking solar power generation last week may end up as an outlier.

“Solar likes sunshine, but it doesn’t like being hot,” Alastair Buckley, a professor of organic electronics at the University of Sheffield, recently told the Daily Mail.

“We’re very unlikely to see any solar records broken this week—simply because it’s so hot and solar panels are less efficient in the heat,” he added.

Even solar panel providers have said that solar output could hit a ceiling as a result of high temperatures.

“The last few days have seen about 10% of Britain’s electricity come from solar. The heat itself, however, brings down the efficiency of the panels slightly, so we don’t expect to see records set,” Chris Hewett, chief executive at trade association Solar Energy UK, told Fortune.

Solar panels are not the only energy source that can see electricity output reduced by high temperatures. In France, electricity output from two nuclear power plants was reduced last week owing to the heat wave raising the temperature of the water in the nearby Rhône River used to cool nuclear reactors.

“Thermal power plants—that’s coal-fired power plants, gas-fired power plants, and nuclear power plants—need huge amounts of cooling water,” Pier Stapersma, a senior researcher with the Clingendael International Energy Programme in the Netherlands, told Fortune.

Stapersma added that, while more and more European power plants have been built in coastal areas in recent years to take advantage of seawater for cooling, many are still located inland, where high temperatures are warming water reserves much faster.

“Any thermal plant that is inland and depends on freshwater rivers gets easily into trouble during hot periods,” Stapersma said. “That is indeed a significant element.”