It seems obvious that the hotter and sunnier the day, the more your solar panels must be making. The first half is right and the second is not. Panels are driven by light, not heat, and in fact they produce slightly less for a given amount of sunlight when they get very hot. That sounds like bad news for a heatwave, but summer is still comfortably your best season for generation. Here is how that apparent contradiction works.
The short answer. Solar panels produce a little less per unit of sunlight when they get hot. Output falls by roughly 0.3 to 0.4 per cent for every degree the panel sits above 25C, and in strong sun a panel can reach 50 to 65C, trimming maybe 10 to 15 per cent off its peak. But summer still gives by far the most generation overall, because the long days and high sun more than make up for the heat penalty. A panel's ideal is a cold, bright, sunny day, not a baking one.
Panels want light, not heat
A solar panel makes electricity from light, not from warmth, and these are not the same thing. You can have brilliant light on a cold, clear day, and you can have fierce heat under a hazy sky. What the panel converts is the light landing on it; the heat is, if anything, a mild hindrance. This is the point that catches people out, because we naturally lump "hot" and "sunny" together, when for a solar panel they pull in opposite directions. The solar basics guide covers how the panels turn that light into power; this one is about why the temperature of the panel matters separately.
Why heat reduces output
Every panel is rated at a standard cell temperature of 25 degrees, and its performance is measured against that. As the cells heat up beyond that point, the voltage they produce drops slightly, and so does their output. The figure is set by the panel's temperature coefficient, typically a loss of around 0.3 to 0.4 per cent for each degree above 25. That sounds trivial until you realise how hot a panel gets in full sun: sitting in direct sunlight on a still day, the cells can reach 50 to 65 degrees, well above the 25 they were rated at, so the accumulated penalty can be 10 to 15 per cent off the nameplate figure. The panel is still generating plenty; it is just generating a little less than its sticker would suggest, because it is running hot.
But summer still wins, easily
None of this means a heatwave is bad for your generation. The heat penalty is a small percentage knocked off a very large summer output, and that output is large because summer brings long days and a high sun, which together deliver far more light over a day than winter ever can. So your panels still make their most in the bright half of the year, heat penalty and all; a hot July day comfortably beats a cold December one despite the temperature working against it. The right way to think about it is that summer gives you a big cake with a thin slice taken off the top for heat, while winter gives you a much smaller cake to begin with.
| Conditions | Generation |
|---|---|
| Cold and bright, full sun | Best per hour, panels near their rated output |
| Hot and sunny, heatwave | Slightly below peak, but long days mean a high daily total |
| Warm but overcast or hazy | Much lower, because there is less light to convert |
| Short winter days | Lowest total, little daylight however clear |
Light is what counts, and length of day with it. A cold sunny day gives the best output per hour; a hot sunny day gives the best total because the sun is up for so long.
The ideal conditions
If you wanted to design a perfect solar day, it would be cold and brilliantly sunny, the cells kept cool by the chill air while bathed in strong light. Those crisp, bright spring days, clear skies and a low temperature, are when panels often punch above their rated figure, which surprises owners who expected the height of summer to be the peak. The very hottest, stillest days of a heatwave are not quite the best per hour, because the panels are running hot, though the sheer length of daylight still makes them strong days overall.
Is there anything to do about it?
For a homeowner, almost nothing, and that is fine. Panels are mounted with an air gap behind them precisely so air can circulate and carry some heat away, which is part of why a roof array copes better than panels laid flat against a hot surface. Beyond ensuring nothing blocks that airflow, there is no day-to-day action worth taking; the heat penalty is a known, modest fact of how panels work, already accounted for in any sensible generation estimate. The more useful summer thought is what to do with all that long-day generation: a solar battery stores the midday surplus for the evening, and timing the washing, dishwasher and any car charging into the sunny middle of the day soaks up power you would otherwise export cheaply, which is where the real value lies, as the is solar worth it guide explains.
The bottom line
Solar panels produce slightly less per unit of sunlight when they are hot, losing around 0.3 to 0.4 per cent a degree above 25, so on a baking day a panel running at 50 to 65 degrees gives maybe 10 to 15 per cent below its rated peak. But this is a thin slice off a big summer total: the long days and high sun mean summer is still by far your best generation season, and a cold, bright day is what a panel likes best per hour. Heat is a minor handicap, not a problem, and nothing for an owner to lose sleep over.