All solar cells have a temperature coefficient.
As a solar panel increases in temperature, the power output of the solar panel decreases.
Generally, monocrystalline solar cells have a temperature coefficient of -0.5%/degC.
This means a mono solar panel will lose half of one percent of its power for every degree the temperature rises.
Solar panels are all rated at 25degC, however, when solar panels are installed on a roof, they generally reach much higher temperatures.
Lets say a 250W monocrystalline solar panel installed on a roof is at 65degC.
The solar panel’s power loss can be calculated as follows:
65degC – 25degC = 40degC
40degC x -0.5% = 20%
Therefore panel power loss = 20% x 250W = 50W
Therefore panel power = 200W