Why Do Solar Panels Lose Efficiency in Hot Weather?

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 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.

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.

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.

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%.

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.

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 it is chosen for yield rather than 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.

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.