Temperature coefficient
The percentage change in a PV module power, voltage or current per degree Celsius of cell temperature away from 25 °C.
Solar cells lose efficiency as they heat up, mainly because the bandgap narrows and open-circuit voltage falls. Datasheets list three coefficients: for maximum power (γ, the important one), open-circuit voltage (β) and short-circuit current (α, slightly positive).
Typical power coefficients γ by technology:
- Multicrystalline silicon: −0.39 to −0.42 %/°C
- PERC monocrystalline: −0.34 to −0.37 %/°C
- TOPCon: −0.29 to −0.32 %/°C
- Heterojunction: −0.24 to −0.26 %/°C
- CdTe thin film: −0.28 to −0.32 %/°C
Example: on a 35 °C day with 1,000 W/m², a module with NOCT of 44 °C reaches about 65 °C. With γ = −0.35 %/°C it delivers 14% less than its STC rating; an HJT module at −0.25 %/°C loses 10%. Over a year, thermal losses are typically 2–4% in temperate climates and 6–10% in hot deserts.
Mounting matters as much as cell technology: roof-integrated modules with no rear ventilation can run 15–20 °C hotter than open-rack ground mounts. Voltage coefficients are also used to size string length for the coldest expected temperature.
Related terms
All terms →- NOCT
Nominal Operating Cell Temperature: the temperature a PV module cells reach under 800 W/m², 20 °C ambient air and 1 m/s wind on an open rack.
- Standard Test Conditions (STC)
The reference conditions for rating PV modules: 1,000 W/m² irradiance, 25 °C cell temperature and an AM1.5G solar spectrum.
- Heterojunction (HJT) solar cell
A cell that sandwiches an n-type crystalline silicon wafer between ultra-thin amorphous silicon layers, giving very high voltage and a low temperature coefficient.
- Performance ratio
Ratio of actual AC energy output to the theoretical output implied by plane-of-array irradiation and nameplate capacity; a location-independent quality metric for PV plants.
- String (PV)
A group of PV modules wired in series so that their voltages add up while the current stays that of a single module.