Solar modules like light, but not heat. As cell temperature rises, output drops slightly – a physical effect experts call the temperature coefficient. That's why cool, clear spring days often deliver surprisingly high yields, while a heatwave in midsummer doesn't automatically bring the annual record.
Key points in brief
- Solar cells lose slightly in output as temperature rises – that's normal physics, not a defect.
- The temperature coefficient on the data sheet states how strong this effect is per degree above 25 °C.
- Cool, clear spring days are therefore often among the highest-yielding days of the year.
- Good rear ventilation of the roof noticeably mitigates the effect.
Why does a solar module's output drop in heat?
Because as cell temperature rises, the solar cell's electrical properties deteriorate slightly – the voltage drops a little while the current stays nearly the same. Overall this results in a small but measurable loss of output. That's counterintuitive, because in everyday life sun and warmth are usually thought of together, but for photovoltaics it's primarily the amount of light that counts, not the heat.
Important for context: this effect concerns the cell temperature, not the outside temperature. A module in blazing sun can heat up well beyond the ambient temperature even on a cool day. That's exactly why the reference point in data sheets is always the cell temperature of 25 °C, as defined under standard test conditions.
What does the temperature coefficient on the data sheet say?
The temperature coefficient describes by how many per cent a module's output falls per degree Celsius above 25 °C cell temperature. For most modules common today, this value is in the range of a few tenths of a per cent per degree – the exact figure differs by cell technology and manufacturer and is always in the respective data sheet.
This figure is already taken into account in a system's yield planning. Anyone choosing a module with a particularly low temperature coefficient benefits from a somewhat smaller loss of output on hot summer days. For the fundamental choice of a standard module for a single-family roof, though, this difference is rarely decisive – we recommend, when in doubt, orienting yourself by the specific data sheet rather than blanket statements.
Why do cool spring days sometimes deliver more yield than midsummer?
Because on a cool, clear spring day two factors combine favourably at once: high, undimmed irradiance and a comparatively low cell temperature. That combination often produces a very high output level. On a hot July day with the same irradiance, the modules heat up more, which costs part of the potential again.
Is that a cause for concern if the app shows no new record in midsummer? No – it belongs to a solar system's normal annual pattern and is factored into yield planning from the start. Viewed over the whole year, it's clearly balanced out by the much longer and more intense sunshine duration in summer.
| Weather situation | Irradiance | Cell temperature | Output level |
|---|---|---|---|
| cool, clear spring day | high | moderate | very high |
| hot, clear summer day | high | high | high, but reduced |
| overcast day | low to medium | low | low to medium |
| clear winter day | low | low | medium (short sunshine duration) |
Classification from practice, as of: July 2026 — the figures are to be understood qualitatively; exact values depend on the respective module and location.
What can homeowners do about the heat effect?
Strictly speaking, little – and that isn't necessary either. The most important lever is the mounting type: an on-roof installation lets air circulate on the module's underside and therefore cools noticeably better than an in-roof solution where the modules are integrated flush into the roof and rear ventilation is restricted. Anyone weighing the two variants finds the details in the guide In-roof vs. on-roof mounting.
For systems already installed there is no need to act: actively cooling the modules would be technically elaborate and economically pointless. The effect is priced in as soon as a system has been planned professionally.
In our customer conversations in the Zurich region, homeowners in midsummer regularly ask why the monitoring app shows less on a hot day than on a cool day in May. After a brief look at irradiance and temperature curve, it can almost always be explained plausibly – it's practically never a cause for concern.
Frequently asked questions
Is a drop in output in heat a defect?
No. The slight decrease in output at high cell temperature is a known physical effect that occurs in every crystalline solar module. It's documented in the data sheet's temperature coefficient and already accounted for in yield planning – so there is no defect.
Do I have to actively cool my modules?
No, that's neither necessary nor usual. Good rear ventilation, as provided by a properly executed on-roof installation, is entirely sufficient as passive cooling. Active cooling systems are not economically worthwhile for private single-family systems.
Why does May sometimes produce more than July?
Because cool, clear spring days often combine high irradiance with lower cell temperatures. That constellation can briefly deliver higher output peaks than a hot summer day. Calculated over the whole year, summer still remains higher-yielding because of the longer sunshine duration.
Does the colour of the module play a role?
There's a certain influence: darker modules absorb somewhat more heat and therefore warm up slightly more than lighter ones. In practice, though, this difference is small and rarely the decisive criterion for choosing a module.
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Sources: EnergieSchweiz, Swissolar, manufacturer documentation.
Last updated: 9 July 2026 · Author: ecoEn editorial team
