With an amber heat-health alert in force and temperatures nudging 35C across much of the country, it looks like the perfect moment for a solar array: blazing sun, clear skies, the longest days of the year. Yet there is a detail that catches out anyone watching their generation figures. On the very hottest days, rooftop solar panels produce less than you would expect. It is not a fault. It is physics.
Why extreme heat cuts output
A solar panel is rated in the lab at a reference temperature of 25C, what the industry calls STC, or standard test conditions. That is the temperature at which it delivers the power printed on its spec sheet. For every degree above 25C, a panel loses roughly 0.3 to 0.4% of its efficiency.
The catch is that the panel runs far hotter than the air around it. Under a heatwave sun, a dark surface baking for hours can reach 65 to 70C. At those temperatures output can fall by 10 to 20% compared with the same amount of light on a cool day. The sunshine is there in abundance, but the cell converts it into electricity less efficiently precisely when it is hottest.
The peak-demand paradox
This is where it gets awkward. The middle of a 35C afternoon is when air conditioning and fans run flat out and electricity demand hits its peak. It is the same moment when every panel is delivering a little below its theoretical best.
Across the grid the two effects stack up: soaring demand and heat-dented solar yield push wholesale prices higher in the early afternoon, exactly when cooling loads are heaviest and the system is most stretched. It is worth understanding how electricity prices move on days like these. The hour of greatest need is also the hour of greatest strain on prices.
What it means for your bill
There is nothing to adjust on the panels themselves: the heat loss is well understood and already factored in when a system is designed. An array fitted with a decent air gap behind the modules runs cooler and suffers slightly less, but that is down to installation, not day-to-day use. Your real room for manoeuvre lies in when you use power, not in what the roof produces.
Under the Ofgem price cap for 1 July to 30 September 2026, electricity costs about 26p per kWh plus a 57p daily standing charge, so every unit pulled from the grid at peak times counts. Shifting flexible jobs (the washing machine, the dishwasher, charging your devices) to the hours the panels are actually generating improves your self-consumption, meaning the share of energy you use directly rather than buying back later. Lining up your usage with the sunny hours is the simplest way to make the system pay. Our guide on how to save electricity is a good place to start.
This heat does not make solar pointless: over summer it still generates plenty, thanks to the long days and strong light. It simply overturns the starting assumption. Hotter does not mean more, and the difference on your bill comes down to how you spread your usage across the day. Understanding when and how much you actually use is worth more than any extra degree on the thermometer.