Manual cleaning is performed by brushing and rinsing the panels with brooms and water. With PV systems that are placed on rooftops, workers are at risk due to height and due to having to work on a wet roof.
Quality of cleaning varies and depends on the worker skills, motivation and on the equipment they use. When the panel is wet it is hard to tell whether it was properly brushed and if all the dirt was removed. Stains or a heavy dirt coating may be only partially removed with remains left on the glass after the panel dries. Mechanical cleaning allows absolute control over panel brushing and complete removal of dirt takes place systematically.