The formulas
Cell temperature ≈ Air temperature + (NOCT − 20) ÷ 800 × Sunlight (W/m²)
Output = Pmax × Sunlight ÷ 1,000 × (1 + Coefficient × (Cell temperature − 25))
Worked example
A 200 W panel with a coefficient of −0.37 %/°C and NOCT of 45 °C, in full 1,000 W/m² sun on a 30 °C day: the cells reach about 61 °C, losing about 13%. Output is roughly 173 W before wiring and controller losses.
Keeping panels cool
- Leave an air gap under roof-mounted panels. Panels glued flat to a van roof run noticeably hotter.
- Choose panels with a low temperature coefficient for hot climates.
- Cold, bright days are when panels perform best, but also when their voltage is highest. Check with the MPPT string voltage calculator.
Frequently asked questions
How much power do solar panels lose in heat?
About 0.3–0.45% per °C above 25 °C cell temperature. On a hot summer day that's typically 10–20%.
What is NOCT?
Nominal operating cell temperature: the cell temperature at 800 W/m² sun, 20 °C air and a light breeze. It shows how hot a panel runs in practice.