Leaves, bird poop and airborne particles (from dirt and pollen) all dirty your solar panel investment. Dirty panels mean power loss -- grime and debris reduce the ability of the solar array to perform at full capacity. A solar array is made up of several solar modules wired together, and each solar module is made up of solar cells all mounted collectively in a frame. Solar cells, in turn, are made of semiconductor materials, such as silicon. One side of a solar cell is positive, the other negative. When sunlight hits a solar cell, it excites the electrons in the semiconductor material, and it's this energy that can be captured as electricity. If the cells aren't clean, then less sunlight is able to be absorbed and less electricity is generated.
The potential energy loss depends on the level of filth your solar panels have accumulated. Energy loss could reach 25 percent, but some reports bring the number as high as 30 percent for consumers who never clean their system [source: Heliotex].
Homeowners with small solar systems may choose elbow grease as their preferred cleaning method. Solar panels, like windows, can be cleaned with warm water and dishwashing soap to remove any dirt or residue. When you wash them, you're washing the glass surface and the frame of the panel.
Sweat equity has its downside, though, and manually cleaning solar panels can come with risks. First, you (or the person who is doing the cleaning) risk personal injury. If you're not comfortable on your roof, consider your personal safety before ascending a ladder. Also, while soap and water won't hurt your solar panels, you still run the risk of damaging them (remember this is an electrical system you're cleaning) or even your roof if you don't know what you're doing. Some things are best left to professionals.
And what about large or industrial-sized solar systems? Should you hire a maintenance crew of panel washers to scale the place, window washer-style? There's always the automatic cleaning system option.