Solar Panel Temperature Derating Formulas

Solar panel temperature formulas: cell temperature from air temperature and irradiance with NOCT, the temperature coefficient and the hot-cell power.

The formulas

Tamb is the air temperature in °C, G the irradiance in W/m², NOCT the nominal operating cell temperature on the data sheet (often 42 to 46 °C), and γ the power temperature coefficient per °C (about −0.0034 for many silicon modules, which is −0.34% per °C). Pstc is the rating at 25 °C and 1,000 W/m². The NOCT rule is an estimate and ignores the wind speed and the way the module is mounted. The voltage also falls with temperature, which matters for the string size, so use the voltage coefficient and the coldest and hottest temperatures for that.

Worked example: 550 W module, 35 °C air, 1,000 W/m²

NOCT = 45 °C and γ = −0.34% per °C.

1. Tcell = 35 + (45 − 20) / 800 × 1000 = 66.25 °C
2. ΔT = 66.25 − 25 = 41.25 °C
3. k = 1 − 0.0034 × 41.25 = 0.8598
4. P = 550 × 0.8598 = 472.9 W

Power at 1,000 W/m² by air temperature

Frequently asked questions

Why does a module lose power when it is hot?

The voltage of a silicon cell falls as the temperature rises, and it falls faster than the current rises, so the power drops.

What is NOCT?

The cell temperature at 800 W/m², 20 °C air and a 1 m/s wind with an open back. It is on the data sheet and gives the rise of the cell above the air.

Does the mounting matter?

Yes. A roof-mounted module with little air behind it runs hotter than a ground-mounted one with free airflow, and loses more power.

Should I use this for the string design?

For the energy yield, yes. For the string voltage limits use the voltage temperature coefficient and the extreme temperatures, as in the solar PV article.

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