Hello, I am Everett Joy from the USA. The surface of the solar panels removes dust, dirt, debris, and other types of contaminants that can reduce their efficiency. This can be done using water, soap, and a soft brush or sponge, or by using specialized washing solar panels cleaning equipment. The frequency of cleaning depends on various factors such as the location, weather conditions, and the amount of dust and dirt in the area. Proper cleaning of solar panels can help to ensure that they operate at their maximum efficiency and last longer.