Fundamentally, from the intensity of the light radiation and the temperature of the solar cells. The intensity of current generated by the panel increases with radiation, and the voltage remains more or less constant. In this sense, the setting up of the panels is of paramount importance (their orientation and inclination in regards to the horizontal), as the values of radiation change along the day according to the inclination of the sun in respect to the horizon.

The increase of the temperature in the cells means an increase of current, but at the same time a proportionally higher decrease in voltage. The total effect is that the power of the panel decreases when its working temperature increases.

A 1.000 W/m2 radiation is able to heat up a panel 30 degrees over the surrounding temperature of the air, which reduces the voltage in 2 mV/(cell*degree) * 36 cells * 30 degrees = 2,16 Volts, and therefore power on a 15%. That is why it is important to set up the panels in a well aired place.