As for cleaning agents, avoid anything abrasive, and harsh chemicals. Remember you are dealing with delicate equipment that just has been exposed to the natural elements. So no need to over do it by scrubbing too hard, since after all, it’s only accumulated dirt you are trying to remove. Water and scrubbing with a sponge is normally enough for maintaining your solar panels. Tougher dirt may require some dish soap.
A bad idea is trying to expedite the process using a pressure washer. This will probably damage the delicate film that covers the panels, and even cause the panel to crack. A garden hose with good flow and not much pressure will do the work.
Since in many areas water tends to be hard, it is a good idea to rinse the panels with soft water. If the panels are not too big, a couple of gallons of distilled water will be enough; if you need more, and don’t want to incur in an extra expense, just collect some rainwater in advance and use it for that purpose.
It also helps to use a squeegee to remove excess water, the reason being that if you let the water just sit and evaporate, calcium deposits will be left on the panel’s surface.
Solar panel maintenance is usually required once or twice a year, but in some areas high humidity combined with airborne dust will call for more frequent cleaning.
Properly maintaining your solar panels is extremely important, not only to prolong their useful life, but also because a dirty panel has its efficiency reduced by up to 25%.