Just like windows, solar panels build up grime, dirt and dust, which can reduce the efficiency of the panels as well as potentially damaging the solar panels or fittings. This build up can come from wildlife, plants or simply just the elements and can include dust, pollution, debris from building sites, bird droppings and salts from sea mists(this is especially prominent in coastal areas) as well as many other contaminants. Sea salts collect a lot quicker than other substances and therefore installations will need cleaning more often to ensure optimum efficiency and to stop corrosion of the frames and fittings.
Due to the angle of solar panels (skywards facing) they are prone to collect all the dirt and dusts mentioned above. Rain water will clean the panels slightly but rain also contains some impurities that can leave stains on your solar panels.
Solar panels with a lower tilt will require more frequent cleaning as they have a higher chance of collecting debris.
Although Solar panels have a self-cleaning surface this usually does not keep solar panels clean enough to maintain optimum efficiency.
Another reason to clean your solar panels is to ensure they are kept under warranty with regards to both efficiency output and parts. This will be stated in the pack that you received from your installer upon completion, if you cannot find your pack or the information within the pack the manufacturers will be able to advise you on this.
Be aware that solar panel warranties are not valid unless there is proof of regular professional cleaning.