How solar panels work, in theory and in practice

We present an analysis of the functionality of an array of monocrystalline silicon solar panels over a 22 month period. For simple geometrical reasons, one expects the solar power produced to be linearly proportional to the cosine of the incidence angle of the Sun's light on the panels. This can be demonstrated with high accuracy (root-mean-square scatter of less than one percent), but one must first correct the measured solar power for the dimming effect of the Earth's atmosphere as a function of the atmospheric path length that the Sun's light traverses. We elaborate a simple and robust method for determining the atmospheric extinction term on days with clear sky conditions. There are two situations under which the extinction-corrected data deviate from the linear relation. One is due to the heating of the panels during daylight hours, resulting in their being less efficient. The other is manifested as an increase of power that occurs when the sky is partially filled with reflective clouds. This is the phenomenon of cloud enhancement.