From the day of placement a layer of fine dust forms on the panels, which is not rinsed off by rain. This layer may be limiting the amount of incident light.
When it rains, dirt slips off easier from panels that are installed at a steeper angle. One could say that the smaller the slope, the faster the panels get dirty.
Besides cleaning, the plant is visually checked for defects. All observed defects are extensively reported.
In cooperation with an official investigative body, Solar Clean has done extensive testing related to the cleaning of solar panels.