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WattCurious

Why Do Solar Panels Lose Efficiency in Hot Weather?

Updated 8 October 20267 min readSolar Energy

Heat costs voltage, not light. Warming a silicon cell narrows its band gap slightly and multiplies the thermally generated carriers inside it, which drags the operating voltage down considerably faster than the current creeps up. Power is the product of the two, so it falls. The loss is entirely reversible, and the datasheet states its size.

Key takeaways

  • A hot module is not receiving less light. It is converting the same light at a lower voltage, which is a different problem with different symptoms.
  • Three temperature coefficients appear on every datasheet — for maximum power, open-circuit voltage and short-circuit current. Only the first two matter much.
  • Rated power is measured at a cell temperature of 25 °C (77 °F), which a sunlit module almost never sits at.
  • The single biggest lever an installation has is airflow behind the module, and it is decided at mounting time.
  • Heat depresses voltage while soiling and shade depress current. That difference identifies the cause without removing anything from the roof.
On this page
  1. Heat is not shade
  2. What warming does inside the cell
  3. Reading the three coefficients
  4. The gap between the label and the roof
  5. What actually changes module temperature
  6. Telling heat from a fault

Heat is not shade

The first thing to establish is what heat does *not* do. It does not reduce the light arriving at the cell. A module at 65 °C (149 °F) in full sun is receiving exactly as many photons as an identical module at 25 °C (77 °F) in the same irradiance.

This matters because the two most common causes of lost output — soiling and partial shading — work by blocking light, and they produce a fall in *current*. Heat produces a fall in *voltage*. That is the whole diagnostic, and it is available from any inverter display.

It also explains a result that surprises people: cloudy weather and hot weather affect a system in opposite ways. Cloud takes light away while keeping the module cool; a clear hot day delivers maximum light to a module working at its least efficient. The first case is covered in do solar panels work on cloudy days.

What warming does inside the cell

Two effects run at once inside a heated silicon cell, and they pull in opposite directions.

The band gap narrows. As the lattice expands and vibrates harder, the energy needed to lift an electron into the conduction band falls slightly. A few more long-wavelength photons can now be absorbed, so short-circuit current rises — by a very small amount.

Thermal generation multiplies. Heat also creates electron-hole pairs on its own, with no photon involved. That raises the cell's dark saturation current, which is a measure of how readily charge leaks back across the junction. Open-circuit voltage falls as a result — by a much larger amount.

Diagram of a silicon cell at two temperatures. On the cool side, few thermally generated carriers cross the junction and the built-in voltage is high. On the hot side, the band gap is slightly narrower and many more thermally generated electron-hole pairs appear, increasing recombination across the junction and lowering the voltage the cell can sustain.Diagram of a silicon cell at two temperatures. On the cool side, few thermally generated carriers cross the junction and the built-in voltage is high. On the hot side, the band gap is slightly narrower and many more thermally generated electron-hole pairs appear, increasing recombination across the junction and lowering the voltage the cell can sustain.
Heat helps the cell absorb a fraction more light and hurts its ability to hold a voltage. The second effect is much the larger of the two.

Power is voltage times current, so the second effect decides the outcome. This is a property of the semiconductor, not a defect: every silicon module ever made behaves this way. Thin-film technologies generally have a gentler coefficient, one of the real-world differences between monocrystalline, polycrystalline and thin-film panels.

Current-voltage curves for a photovoltaic module at a cool and a hot cell temperature. Both curves start at nearly the same short-circuit current, but the hot curve reaches zero current at a noticeably lower open-circuit voltage, so its maximum-power point sits lower and to the left.Current-voltage curves for a photovoltaic module at a cool and a hot cell temperature. Both curves start at nearly the same short-circuit current, but the hot curve reaches zero current at a noticeably lower open-circuit voltage, so its maximum-power point sits lower and to the left.
The same module at two cell temperatures. Short-circuit current barely moves; open-circuit voltage shifts left, and the maximum-power rectangle shrinks with it.

Reading the three coefficients

Every module datasheet carries three temperature coefficients, each expressed as a percentage change per degree Celsius relative to the value at 25 °C.

What each coefficient tells you
CoefficientSignRoughly how bigWhat it governs
Maximum power, PmaxNegativeThe headline numberHow much energy you lose on a hot day
Open-circuit voltage, VocNegative, larger in magnitudeLarger than PmaxThe highest string voltage the system will ever see — on the coldest morning
Short-circuit current, IscPositiveSmallBarely affects yield; matters for conductor sizing

Signs and relative magnitudes are consistent across silicon modules; the values are product-specific and belong to the datasheet for the module actually installed. The worked figures below use −0.35 %/°C purely as an example.

The arithmetic is deliberately simple. Take the cell temperature, subtract 25 °C, multiply by the Pmax coefficient, and apply that to the rated power:

  • A module at 55 °C is 30 °C above rating. At −0.35 %/°C that is −10.5%.
  • The same module at 70 °C is 45 °C above rating: −15.75%.
A bar showing nameplate power at the top, with successive deductions marked below it: the loss from operating thirty degrees above the rating temperature, then further ordinary system losses, leaving the power actually delivered. The temperature deduction is the largest single step.A bar showing nameplate power at the top, with successive deductions marked below it: the loss from operating thirty degrees above the rating temperature, then further ordinary system losses, leaving the power actually delivered. The temperature deduction is the largest single step.
The temperature deduction is applied to the nameplate figure before any other loss. On a hot roof it is usually the largest single step between the label and the meter.

Note the direction of the Voc coefficient. Because voltage rises as temperature falls, the maximum string voltage a system will ever see occurs on the coldest morning of the year, not the hottest afternoon. String length is checked against that worst case.

The gap between the label and the roof

Rated power is measured at Standard Test Conditions: 1,000 W/m² of irradiance, an AM1.5G spectrum, and a cell temperature of 25 °C (77 °F). The third of those is a laboratory condition, not a description of a roof.

A module in full sun runs hot because most of the light it absorbs does not leave as electricity. Datasheets therefore also quote a figure at nominal operating cell temperature (NOCT, or NMOT in newer standards), measured under a defined irradiance, ambient temperature and wind speed to represent a realistic mounting.

Chart of module temperature against ambient air temperature for three mounting arrangements: a well-ventilated rack rising modestly above ambient, a typical roof mount with a gap rising further, and a module mounted flush against a surface rising the most. All three sit well above the twenty-five degree rating temperature in full sun.Chart of module temperature against ambient air temperature for three mounting arrangements: a well-ventilated rack rising modestly above ambient, a typical roof mount with a gap rising further, and a module mounted flush against a surface rising the most. All three sit well above the twenty-five degree rating temperature in full sun.
Illustrative. In full sun every mounting runs well above air temperature; how far above is set by how easily heat can leave the back of the module.

What actually changes module temperature

A module loses heat three ways: radiation from both faces, convection to moving air, and conduction into whatever it is attached to. Installation decides how well the second and third work.

  • The gap behind the module matters more than anything else. A ventilated rack lets convection carry heat off the rear surface; a module laid flush against a roof surface has nowhere to send it.
  • Wind exposure is free cooling and entirely site-dependent. It is also why the same module runs cooler on an exposed rack than in a sheltered corner.
  • The surface beneath re-radiates heat upward. A dark membrane roof is a warmer neighbour than a light or ventilated one.
  • Tilt aids convection slightly by helping warm air rise off the back, though the angle is chosen by what maximises yield over the year rather than by cooling.

None of these can be changed after commissioning without rebuilding the mounting, which is why the temperature question belongs in the design conversation rather than the troubleshooting one.

Telling heat from a fault

Heat produces a characteristic signature, and it is easy to separate from the alternatives.

Two diagnostic cases compared. On the left, a hot module: current is normal while voltage is depressed, and output recovers as the module cools. On the right, soiling or shade: voltage is normal while current is depressed, and output does not recover with temperature.Two diagnostic cases compared. On the left, a hot module: current is normal while voltage is depressed, and output recovers as the module cools. On the right, soiling or shade: voltage is normal while current is depressed, and output does not recover with temperature.
One pair of readings separates the two. Heat takes voltage; anything blocking light takes current.
  1. Check which quantity has fallen. Voltage down with current normal is thermal. Current down with voltage normal is optical — soiling, shading or a bypassed cell group.
  2. Watch the daily shape. A thermal loss deepens through the afternoon as the module heats and eases as it cools, even while irradiance is steady.
  3. Compare seasons at equal irradiance. If a clear winter day at the same irradiance outperforms a clear summer one, the system is behaving exactly as designed.
  4. Rule out the rest. If neither pattern fits, the cause is elsewhere, and the ordered approach in why a system produces less power than expected covers the remaining candidates — including hail damage that leaves the glass intact.

The useful mental correction is this: a hot climate is not a bad place for solar. It is a place where a given module delivers a smaller fraction of its nameplate rating while receiving far more sunlight than a cool cloudy one — and the sunlight wins by a wide margin over a year.

Frequently asked questions

Do solar panels work better in cold weather?

They convert light more efficiently when cold, yes. A crisp, clear winter day can produce the highest instantaneous power a system ever records. Over a year, though, a hot sunny climate still delivers far more energy than a cold cloudy one, because irradiance and daylight hours matter more than the conversion penalty.

How much output does heat actually cost?

Multiply the temperature difference from 25 °C by the maximum-power coefficient on the datasheet. A module running 30 °C (54 °F) above its rating temperature with a coefficient of −0.35 %/°C is about 10.5% down before any other loss. Your module's coefficient is the one to use; they differ between technologies and products.

Will hosing panels down improve output?

Briefly, and it is not worth doing. Evaporative cooling fades within minutes, mineral deposits are left behind as the water dries, and cold water on hot glass risks thermal shock. Cleaning is worth doing for soiling, which is a different problem.

Is heat damaging the panels?

Ordinary operating heat is not. Output recovers completely as the module cools, and this reversible loss should not be confused with degradation, which is permanent and takes years. What heat does accelerate is the ageing of materials such as the encapsulant and the junction box.

Why does my system peak in spring rather than midsummer?

Because spring often combines strong irradiance with cool air. Midsummer brings more sunlight but hotter modules, and the two effects partly cancel. Peak instantaneous power and peak seasonal energy do not have to arrive in the same month.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

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Last reviewed 8 October 2026. How we research and review