A solar panel typically consists of solar cells covered by a protective glass coating, and just like your car windshield - the longer it is outside - the more likely it will get dirty and need to be cleaned.
HOW MUCH ELECTRICITY DO DIRTY SOLAR PANELS LOSE?
Experts agree that dirty solar panels don't produce as much power as clean panels. That loss may range as high as 25% in some areas according to the National Renewable Energy laboratory. Individual dealers have reported losses as high as 30% for some customers who fail to ever clean their panels.