Solar modules accumulates dust, dirt, leaves, bird dropping and other detruis over time. These materials block some of the sunlight from striking the PV cells (or thermal absorber in the case of a solar thermal system). For roof mounted systems, this blocking of the sun can amount to 5-10% over a years' time, and it is important to wash solar panels about once a year to keep soiling to a minimum.
Light, moderate or heavy deposits of dust, dirt, soot, generally deposited by electrostatic action.