If you have solar panels and are looking ahead to the summer months, you might be wondering whether a heatwave could create more free electricity. This may be to keep an air conditioner or fan running – or to sell surplus electricity back to the grid.
The answer to this question is: there’s both good news and bad news. Longer, brighter days do tend to mean more solar generation overall because sunlight hits your panels for more hours. But, high temperatures can slightly reduce how efficiently solar panels turn sunlight into electricity.
In the UK, that heat penalty is usually modest, and the evidence from Britain’s hottest day on record suggests solar still performs strongly when the country swelters.
How do solar panels perform when it’s hot?
Modern solar panels are designed to operate across a wide temperature range. According to trade body Solar Energy UK, most panels are built to work from around -40C to 85C, but they generally perform best when the solar cells are closer to 25C or below.
That can feel counterintuitive. After all, the hottest days are often the sunniest. But solar panels don’t like heat in the same way humans do. Strong sunlight increases the electrical current a panel can generate, yet as the panel warms up, its voltage drops, and power is essentially current multiplied by voltage. In practice, that means bright sunshine can still deliver great generation, but higher panel temperatures shave a little off the top.
Air temperature vs solar panel temperature
One important detail is that it’s the temperature of the solar cells inside the panel that matters most, not the headline temperature on the weather app.
Solar panels are rated in laboratory conditions (often called “standard test conditions”) at 25C. Out on a roof, panels can run significantly hotter than the surrounding air because they absorb solar radiation. While wind and airflow under the panels can cool them, on a still, sunny day it’s common for the panels themselves to be far warmer than the ambient temperature.
How much power do solar panels lose in a heatwave?
The impact of heat is usually described using a panel’s temperature coefficient – a number on the datasheet that indicates how much the panel’s power output falls as its temperature rises above 25C.
As a rule of thumb, for every degree above 25C, solar panels can lose around 0.34 to 0.5 percentage points of power output, depending on the model and the quality of the cells.
To see what that means in the real world, consider Britain’s all-time temperature record of 40.3C, set on 19 July 2022.
If you used the air temperature alone as a rough guide, the difference between 25C and 40.3C is 15.3C. On that basis, a good modern panel would be operating at roughly 5 per cent below its optimum rating.
In reality, panels can be hotter than the air temperature in full sun. But even at the top end of many manufacturers’ operating ranges – around 85C – the loss implied by typical temperature coefficients is still usually in the region of 20 per cent compared with the 25C test condition.
That sounds large, but it’s worth keeping in perspective. First, it’s a comparison to an ideal laboratory reference point. Second, it’s a “moment in time” penalty at a particular panel temperature, not a promise that your annual solar yield will fall by 20 per cent.
Heat doesn’t just affect solar panels: the inverter matters, too
If you notice your system output dropping more than the panel maths suggests on very hot days, it may not be the panels at all.
Solar systems rely on an inverter to convert the electricity generated by the panels (DC) into the form used in your home (AC). Inverters generate heat while they operate, and many are designed to protect themselves by reducing output if they get too hot, a behaviour sometimes described as thermal derating or throttling.
That’s one reason installers usually try to position inverters in a cool, well-ventilated place out of direct sunlight. Poor airflow around an inverter – or a unit installed in a hot loft space – can make a bigger difference during a heatwave than the panel temperature coefficient alone.
What happened on the UK’s hottest day on record?
Britain’s record-breaking heat in July 2022 provides a useful real-world test.
According to estimates from Sheffield Solar’s PV Live modelling, cited by Solar Energy UK, solar generated 66.9GWh of electricity over the course of 19 July 2022, supplying around 8.6 per cent of the UK’s power needs that day. Over the previous seven days, solar had provided around 9 per cent.
In other words, even on the hottest day the UK has ever recorded, the national solar fleet still supplied a meaningful share of electricity, and the evidence suggests only a small drop-off in performance.
How to tell if your panels are ‘good in the heat’
If you’re comparing solar panels – or just trying to understand your existing system – a few details can help:
- Temperature coefficient (Pmax): the closer this figure is to zero, the less performance falls as panels heat up.
- NOCT (Nominal Operating Cell Temperature): a clue to how warm a panel tends to run under typical real-world conditions.
- Inverter operating temperature / derating behaviour: some inverters reduce output at high temperatures – good siting and ventilation can minimise this.
In summary
Heatwaves can slightly reduce the efficiency of solar panels, mainly because higher temperatures lower panel voltage. But in the UK, the penalty is usually modest, and Britain’s record hot day in July 2022 still saw solar provide a significant share of national electricity.
As the climate warms, UK heat extremes are becoming more likely. That means it’s worth understanding what your system’s numbers really mean and remembering that good design (especially inverter placement and airflow) can matter as much as the panel brand on your roof.
