What is a solar panel temperature coefficient?

A solar panel's power temperature coefficient describes the relative change in output for each degree of cell temperature change. PVWatts uses minus 0.37% per degree Celsius for its standard crystalline-silicon model [S1]. Use your proposed panel's datasheet value, rather than applying that modelling default to every product or using outdoor air temperature. Checked October 2026.

Last checked 19 October 2026

What does a temperature coefficient mean for your site?

A solar panel’s power temperature coefficient is the relative change in electrical output per degree Celsius of cell temperature change. For your site, it explains part of the difference between a panel’s rated output and its performance on a hot roof. Standard test conditions use a module temperature of 25°C [S2].

Check the proposed panel’s datasheet. PVWatts’ current crystalline-silicon modelling defaults span minus 0.35% to minus 0.37% per degree Celsius for its premium and standard categories [S1]. These are representative model inputs, rather than a verified range covering every panel sold in the UK.

How do you calculate the temperature effect?

For a simplified illustration, assume a power coefficient of minus 0.35%/°C and a cell temperature of 45°C:

• Temperature rise above the reference = 45 − 25 = 20°C.
• Relative power change = 20 × −0.35% = −7%.
• An assumed 400-watt reference output becomes 400 × 0.93 = 372 watts.

This is an illustration, not a quote or annual forecast. It holds sunlight constant and excludes other losses. Use how commercial panels work for the wider conversion process.

Why can cold sunny days produce strong output?

At equal sunlight, lower cell temperature generally improves relative efficiency. Hot sunshine also heats panels above the surrounding air; wind and mounting affect their temperature [S2]. This does not mean winter generates more annual energy than summer. Ask for a monthly model, including daylight and irradiance, through the regional yield guide.

A power coefficient alone cannot diagnose a complete shutdown. Request monitoring records and an installer investigation rather than attributing every output drop to normal temperature loss.

What should you do next?

Keep the datasheet with your commercial solar sizing brief. Use the site check to get matched with an installer that covers your area.

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