The weather was too hot for solar panels on Tuesday as soaring temperatures reduced their efficiency.
As the heatwave pushed the mercury above 40C for the first time ever in Britain, solar output remained well below the levels usually reached at peak times in spring.
Modelling data from the University of Sheffield suggests that solar energy provided an average 2.8 gigawatts of power on Tuesday.
Meanwhile in spring, when the weather is cooler and generation peaks, it typically accounts for 3.3 gigawatts, according to Josh Jackman, researcher at The Eco Experts.
Solar panels become less efficient when temperatures rise above 25C, meaning energy generation drops off, with efficiency decreasing by around 0.35 percentage points for every degree above this level.
Professor Alastair Buckley, of the University of Sheffield, said: "We never see peak output in mid summer.
“The temperature of the actual solar cell depends on a combination of the ambient temperature and the radiative heating from the sun and also cooling from wind. We saw cell temperatures of 70 degrees yesterday on our test system. Normally it would be between 40 degrees and 50 degrees."
Tim Dixon, analyst at Cornwall Insight, said: "The efficiency of solar panels is impacted by temperature, with high temperatures above 25 degrees negatively impacting on performance. It is likely that the extreme temperatures have impacted total output levels."
Chris Hewett, chief executive of industry group Solar Energy UK, said the current weather was "good for solar energy generation" but that the heat "brings down the efficiency of the panels slightly".
Mr Jackman said that solar panels would be performing better in a heatwave than during a spell of cloudy weather despite their limitations at higher temperatures, and that the technology would normally achieve an average efficiency of around 85pc in a year.
He said: "Obviously this is incredibly unusual, but they are generating about 20pc of the UK grid's electricity today.
"If it was a cloudy day, they would also be suffering. Losing 5pc of efficiency sounds bad but on a cloudy day you can lose anywhere from 25pc to 66pc depending on how cloudy it is.
"So actually it's better for it to be too hot than for it to be cloudy.
"They are only at maximum efficiency quite rarely. But who among us is ever at maximum efficiency?"