Does heat reduce solar panel performance? What happens in summer

It sounds like a contradiction: solar panels need sunshine, but they don't like heat. As the cell temperature rises, its voltage drops, and so does the power it delivers. That is why, on an August afternoon in Tarragona, overheated panels produce less than their specifications say.

The 25 °C on the label

A panel's rated power (for example, 450 W) is measured under standard test conditions (STC): 1,000 W/m² of irradiance and a cell temperature of 25 °C. On a roof, in the sun, the cell is almost always hotter than that.

How much is lost per degree

According to PVEducation, the leading educational resource on photovoltaics, the maximum power of a silicon cell falls by 0.4–0.5% for every °C of temperature increase [1]. On your panel's datasheet it appears as the "temperature coefficient of Pmax".

The real operating temperature (NOCT)

Manufacturers also state the NOCT: the temperature the module reaches with 800 W/m² of irradiance, air at 20 °C and wind at 1 m/s. Most modules are around 40–45 °C [2]. In the height of a Mediterranean summer, with more radiation and air at 30 °C or more, the cell gets considerably hotter.

What you can do to keep your panels cooler

• Rear ventilation. PVEducation notes that roof integration, which blocks heat exchange at the back, can raise module temperature by 10 °C or more [2]. Let air flow under the panels.
• Keep them clean. Dirty areas or droppings heat up more than the rest and can create hot spots.
• Keep the space underneath clear. Leaves, nests or debris under the structure restrict ventilation.

Take care when cleaning in summer

Throwing cold water on an overheated panel causes a sudden temperature change. First Solar's guidelines say not to apply water more than 20 °C warmer or colder than the module surface, and to clean in low light [4]. That is why, in our solar panel cleaning service in Tarragona, Reus and the rest of the province, in summer we work early in the morning or late in the day.

About the sources: the water quality, pressure and temperature figures in this article come from the cleaning guidelines of First Solar, one of the largest US manufacturers of solar modules, which makes thin-film modules; silicon modules, more common on rooftops, may have slightly different specifications. Each manufacturer publishes its own, which is why at TARRAMED we always follow those of your panels' manufacturer.