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. Pollution, dust, leaves, pollen and even bird droppings contribute in preventing sunlight from reaching the solar cells in your solar panels.
Solar panels should be inspected on a periodic basis (frequency depends on location or the manufacturer's specifications) to remove any debris and dirt and ensure all connections are tight. Generally speaking, dusty areas typically require more frequent inspection.
WHAT OPTIONS ARE THERE FOR CLEANING SOLAR PANELS?
You can self clean your solar panels. Assuming you can reach your solar panels, clean the surface of the panel with warm water and dishwashing soap to remove any accumulation of dirt and grime.
Hire a professional. If your solar panels are mounted on your roof, you may want to hire a professional. A professional has the proper tools to reach high places with less risk of injury. Make sure they are insured and find out how often cleaning is recommended.
In any event, it is wise to protect your investment and having your panels working at their optimum level.