Solar Panels During a Heatwave: Do They Actually Produce More Electricity?
Solar Panels During a Heatwave: Do They Actually Produce More Electricity?
Hook: Solar panels love sunshine — but they don’t necessarily love heat.
When Britain experiences a run of cloudless, exceptionally hot days, it seems obvious that solar panels should be producing electricity at their absolute maximum.
Blue sky. Intense sunshine. Long summer days. Surely this is perfect solar-panel weather?
It is certainly good solar weather — but there is an important complication.
Photovoltaic panels need light to generate electricity, but as the solar cells themselves become hotter, their efficiency falls. A wonderfully bright but very hot August afternoon can therefore produce less peak power than an equally sunny, cooler spring day.
That sounds contradictory until we separate two things that we normally lump together:
sunshine and temperature.
They are not the same thing.
Sunshine Makes Electricity. Heat Does Not.
A photovoltaic panel converts incoming solar radiation into electricity.
The stronger the sunlight falling on the cells, the more electrical power they can potentially produce. Energy Saving Trust describes exactly this relationship: stronger sunshine generally means more generation, although panels can still operate under cloudy conditions.
But the panel is not a solar-thermal collector. We are not trying to make it hot.
In fact, from an electrical point of view, we would ideally like:
very strong sunshine combined with relatively cool solar cells.
That is one reason a clear, bright spring day can sometimes produce surprisingly impressive peak-power readings.
The sunlight may be strong, but the surrounding air is still cool enough to help remove heat from the panels.
During a heatwave, meanwhile, the roof may already be extremely hot before we even consider the additional heating produced by intense solar radiation.
The Temperature on Your Weather App Is Not the Temperature of Your Panels
Suppose the air temperature outside is 30°C.
That does not mean your solar cells are sitting at 30°C.
A dark surface exposed directly to strong summer sunshine can become substantially hotter than the surrounding air.
The actual operating temperature depends on several factors:
- air temperature;
- strength of sunlight;
- wind speed;
- ventilation beneath the panels;
- how close they are mounted to the roof;
- roof colour and construction;
- panel design.
NREL's modelling of photovoltaic systems explicitly considers irradiance, air temperature and wind when estimating cell temperature. It also notes that roof-mounted panels tend to operate hotter than well-ventilated open-rack installations because airflow behind them is restricted.
So on a 30°C or 35°C summer afternoon, the cells themselves might be considerably hotter.
That is where the loss begins to become noticeable.
Why Solar Panels Lose Power as They Get Hot
Solar panels are normally given a rated output under defined laboratory conditions.
One important reference point is a cell temperature of 25°C, combined with an irradiance of 1,000 W/m2.
Real roofs are rarely laboratories.
Manufacturers therefore specify a temperature coefficient showing approximately how much maximum power changes as cell temperature rises.
The exact figure depends upon the particular technology and panel, but crystalline-silicon modules have historically shown power temperature coefficients around -0.4% to -0.5% per degree C, with modern modules varying by design.
Suppose we use an illustrative coefficient of:
-0.4% per degree C
and the cells reach 60°C.
The temperature rise above the reference temperature is:
60 - 25 = 35°C
The approximate temperature-related power reduction would therefore be:
35 x 0.4% = 14%
So, all else being equal, the panel could be producing roughly 14% less power than it would at 25°C under the same level of illumination.
That is quite a difference.
It doesn't mean the solar system stops working.
Far from it.
It simply means that hotter panels convert the available sunlight slightly less efficiently.
The Interesting Comparison: April Versus August
Imagine two completely clear days.
Day One: Bright spring weather
Air temperature: 15°C
Cell temperature: perhaps 35°C
Very good sunshine
Cool breeze
Using our illustrative -0.4%/°C temperature coefficient:
35 - 25 = 10°C
Approximate temperature effect:
10 x 0.4% = 4% reduction
Day Two: Heatwave
Air temperature: 32°C
Cell temperature: perhaps 60°C
Very strong sunshine
Little wind
Temperature difference:
60 - 25 = 35°C
Approximate temperature effect:
35 x 0.4% = 14% reduction
Under identical sunlight intensity, the cooler panel would have the advantage.
That is the crucial qualification.
Identical sunlight is difficult to achieve in the real world.
So Does the Spring Day Actually Generate More Electricity?
Not necessarily.
This is where the story becomes more interesting.
We need to distinguish between:
maximum instantaneous power
and
total energy generated during the day.
A beautifully clear April day might give an unexpectedly high instantaneous peak because the panels remain comparatively cool.
But a clear June or July day can give us:
- an earlier sunrise;
- later sunset;
- more hours of useful generation;
- a higher Sun in the sky;
- potentially greater total solar radiation.
So even if panel efficiency drops during the hottest part of the afternoon, the summer system may still produce more total kWh over the whole day.
We can therefore quite reasonably have this situation:
Cool spring day: higher peak efficiency.
Hot summer day: more total daily electricity.
Those statements are not contradictory.
This Is Something I Can See in My Own Solar System
This is one of those subjects where having your own solar installation turns an abstract scientific idea into a practical experiment.
With my own system of 26 panels arranged across three arrays and a substantial battery, I can look at much more than simply whether "today was sunny".
I can examine the generation curve throughout the day.
On a very bright day I might expect a smooth rise during the morning, a broad period of high production around midday and then a decline towards evening.
But that graph can reveal some surprisingly subtle effects.
Rather than simply asking:
"How much solar did I generate today?"
I can start asking:
"How much was I generating at midday?"
"What was the outdoor temperature?"
"Was there much wind?"
"How does that compare with a similarly clear day in April or May?"
And suddenly the solar installation becomes a small domestic renewable-energy laboratory.
Try the Experiment Yourself
If your solar inverter or monitoring application retains historical data, this is an excellent experiment to conduct at home.
Find two days.
Day A
Choose a very bright, cloudless spring day with relatively low temperatures.
Day B
Choose an equally clear day during a summer heatwave.
Then record:
| Measurement | Cool sunny day | Hot sunny day |
|---|---|---|
| Maximum power | ||
| Total daily generation | ||
| Midday power | ||
| Maximum air temperature | ||
| Wind conditions | ||
| Sunrise/sunset |
If your monitoring system records each separate solar array, you can investigate even further.
You may discover that east-, south- and west-facing arrays behave quite differently as the Sun moves across the sky.
Don't Compare Random Days
There is an important scientific warning here.
If I compare a sunny 15°C April day with a 32°C August day and find that the April peak was higher, I cannot immediately declare:
"I've proved temperature caused it."
There are too many other variables.
The Sun was at a different angle.
Atmospheric conditions were different.
Cloud may have been present even if I did not notice it.
Haze can reduce irradiance.
Wind affects panel cooling.
Shading changes throughout the year.
Dust or dirt on the panels may differ.
The inverter may impose a maximum output.
Even very thin cloud can alter a power curve substantially.
Solar generation therefore provides a wonderful example of a fundamental scientific principle:
Correlation isn't automatically causation.
The more variables we can measure and control, the stronger our conclusion becomes.
Watch Out for Inverter Clipping
There is another complication.
Imagine your panels are theoretically capable of supplying more DC power than the inverter is designed to convert at one moment.
Once the inverter reaches its maximum output, the graph may flatten.
This is commonly called clipping.
If that happens, a cool spring day might not appear to produce a higher peak because the inverter has already placed a ceiling on what you can see.
The extra capability of the panels is effectively hidden.
So if you see a beautifully flat top to your generation graph, don't immediately assume that the panels just happened to produce exactly the same power for several hours.
You may be looking at an inverter limit.
Could Cooling Solar Panels Improve Their Output?
Technically, yes.
If panels operate more efficiently when cooler, then reducing their temperature can increase electrical output.
But that does not mean we should rush outside with garden hoses.
Using treated drinking water to cool domestic solar panels would generally make little environmental or financial sense, particularly during a drought or heatwave.
Pumps, plumbing, maintenance and water consumption could easily undermine the relatively modest benefit.
Passive cooling is much more interesting.
Good airflow behind panels helps heat escape naturally.
That is why mounting arrangements matter.
There have also been numerous research projects exploring specialised cooling systems, photovoltaic-thermal systems and other approaches where waste heat is deliberately captured.
But for a normal household installation, the sensible approach is normally much simpler:
give the panels reasonable ventilation and let them get on with producing electricity.
Wind Can Be a Solar Panel's Friend
This introduces another interesting observation.
A hot, perfectly still day may actually be slightly less favourable electrically than a sunny day with a decent breeze.
The breeze removes heat from the panels.
It is one reason merely looking at the air temperature doesn't tell us the complete story.
A sunny 25°C day with good airflow may potentially provide cooler cell temperatures than a still 25°C day.
NREL's PV models include wind precisely because module temperature depends partly on heat transfer to the surrounding air.
Solar generation is therefore not just about sunshine.
It is an interaction between light, heat, airflow, installation and electronics.
Heatwaves May Increase Solar Electricity Demand Too
There is a wider Going Green point here.
Britain's electricity demand profile may gradually change as hotter summers become more common.
Historically, our biggest domestic-energy concern has been keeping warm in winter.
Increasingly we may also need electricity for:
- fans;
- heat pumps operating in cooling mode;
- refrigeration;
- home-office cooling;
- ventilation;
- dehumidification.
There is something rather useful about solar generation in this respect.
The days when cooling demand becomes greatest are often also days when solar generation is strong.
The match is not perfect — particularly during the evening after solar output has declined — but solar and battery storage can potentially shift some of that daytime generation into the period when households still need electricity later.
Energy Saving Trust similarly highlights battery storage as one way households can make greater use of the electricity their panels generate rather than relying simply on instantaneous consumption.
Perhaps Heat Isn't the Question We Should Be Asking
When somebody says:
"It was 35°C today, so your solar panels must have produced loads of electricity."
the scientifically correct response is:
"Probably — but because it was sunny, not because it was hot."
In fact, the heat was working slightly against them.
What we really want to know is how much solar radiation reached the panels, for how long, and at what operating temperature.
That is a much more useful way of thinking about solar electricity.
Turn Your Solar Panels Into a Science Experiment
For anyone with a monitored solar installation, I think this is worth investigating rather than merely reading about.
During the next spell of very hot weather, save your generation graph.
Then find one of those spectacularly bright but chilly days next March, April or May.
Compare them.
Look at:
- midday output;
- maximum output;
- total daily generation;
- outside temperature;
- cloud;
- wind;
- length of daylight.
If you have an infrared thermometer or thermal camera, there is another experiment waiting to be done: measure how hot the panels actually become compared with the surrounding air — safely and from an appropriate position, without climbing onto the roof.
You may find that the hottest day is not the day when the system achieves its highest instantaneous efficiency.
Conclusion: Solar Panels Love Light, Not Heat
The simple statement that "solar panels work better in hot weather" is wrong.
The equally simple statement that "solar panels work badly when it is hot" is also misleading.
The truth is more interesting.
Strong sunlight increases generation.
High cell temperatures reduce photovoltaic efficiency.
Summer gives us long days and abundant solar energy, so total daily generation can still be excellent even while the panels are suffering a modest heat-related performance penalty.
That is why a cool, brilliantly sunny spring day can sometimes produce an impressive peak — while a scorching summer day may still win comfortably on total kWh generated.
And it is another example of why Going Green becomes much more interesting when we stop treating our homes as collections of appliances and start treating them as systems we can actually measure and understand.
Solar panels love sunshine.
They just prefer their sunshine served cool.
A Useful Calculation for Readers
For a rough estimate:
Temperature-related power change (%) = temperature coefficient x (cell temperature - 25°C)
For example:
Temperature coefficient = -0.4% per °C
Cell temperature = 55°C
55 - 25 = 30°C
30 x -0.4% = -12%
So the estimated output at that irradiance would be about 12% below the equivalent output at the 25°C reference temperature.
This is illustrative rather than a substitute for the manufacturer's actual temperature coefficient.
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