Here's a fact that surprises many people: solar panels produce less power when they get hot. Their rated wattage is measured in a lab at 25°C, but out in the sun a panel's cells can easily reach 60–70°C — and every degree above 25 shaves a little off the output. This calculator shows exactly how much, so you know the real power to expect on a hot day.
How the calculation works
The formula is straightforward:
Power lost (%) = temperature coefficient × (cell temp − 25°C)
The temperature coefficient is a negative number from the panel's datasheet, typically around −0.4%/°C. So if the cell reaches 60°C, that's 35°C above the 25°C rating, and at −0.4%/°C the panel loses 35 × 0.4 = 14% of its power. A 450 W panel effectively delivers about 387 W in that moment.
What is the temperature coefficient?
The temperature coefficient of power (Pmax) tells you how sensitive a panel is to heat. It's always negative, usually between −0.3% and −0.5% per °C. Closer to zero is better — a panel at −0.30%/°C loses far less in heat than one at −0.50%/°C. If you live somewhere hot, this single datasheet number is worth paying attention to when choosing panels.
Why cell temperature is higher than air temperature
The number that matters is the cell temperature, not the air temperature — and the cells run much hotter than the surrounding air because they're absorbing sunlight. On a 35°C day, roof-mounted panel cells can sit at 60–65°C. Mounting style matters a lot here: panels with an air gap behind them run cooler than those flush against a hot dark roof.
How to reduce the loss
You can't stop panels heating up, but you can limit the damage: leave an air gap for airflow behind the panels, choose panels with a low (near-zero) temperature coefficient, and avoid dark, unventilated mounting surfaces. Good airflow alone can drop cell temperature by several degrees and recover a meaningful slice of the lost power. Our guide on solar panels in hot climates goes deeper on this.
Related tools
Temperature also affects string voltage, which matters for wiring panels in series — see the string sizing calculator. To estimate overall production including losses, use the energy production calculator, and to size an array from scratch, the panel calculator.
Frequently asked questions
How much power do solar panels lose in heat?
Roughly 0.3–0.5% per °C above 25°C. On a hot day cells can hit 60–70°C, so a −0.4%/°C panel can lose 15–18% of its rated power.
What is the temperature coefficient of a solar panel?
It's how much output changes per °C above 25°C — a negative number, typically −0.3% to −0.5%/°C. Closer to zero is better. Find it on the datasheet.
How do I reduce temperature power loss?
Leave an air gap for airflow, pick panels with a low coefficient, and avoid dark unventilated surfaces. Airflow alone can recover several percent.