The best weather for solar panels is cold and sunny. When it gets too hot, the panels start to lose efficiency. This is what the temperature coefficient shows — how much output the panel loses for each degree after 25 °C at which it is tested. Usually, the coefficient varies between −0.3%/°C and −0.5/°C.
Let’s set an example. The Aptos Solar 365 W
panel has a temperature coefficient of −0.36%/°C. The solar panel’s temperature is always higher than the air ambient temperature. On a sunny day Aptos module heats up to 44 °C, which is marked as Nominal Operating Cell Temperature (NOCT) in the datasheet. Let’s see how much output it provides in these circumstances:
365 W − (44 °C − 25 °C) × (365 W / 100 x 0.36%) = 365 W − 24.89 W ≈ 340 W
REC and Panasonic engineers use heterojunction cell technology that gives their panels lower temperature coefficients — around −0.25%/°C.Time of year.
This one is pretty straightforward — there is just less sunlight in the winter and fall compared to spring and summer. The irradiance falls and so does the efficiency. In June-July panels generate about 50% more energy than in December-January.