Always use non abrasive cleansers on your solar panels. An abrasive cleaner can scratch the surface of the panel and cause permanent damage and affect how well the solar panel works. There are commercial cleaning solutions to use or you can use a pool skimmer with a very soft cloth on it.
If your roof cannot to support the weight of photo-voltaic solar panels, you can save a lot of money by investing in thin film solar panels. These panels are very light and flexible, which makes the installation much easier and affordable. Their higher price is also justified by their efficiency.