After a solar PV system is installed very little is ever needed in maintenance. In fact if you have a grid tied solar system that has no batteries, you essentially have a system with no moving parts and no scheduled, maintenance.
If you have batteries then you add some maintenance expenses as batteries will need replacement periodically, but still we are talking about no moving parts and low maintenance. So basically the only aspect of the system that you should assume will need replacement in the future is the inverters. They may need to be replace in 15 years at a cost of a few thousand dollars, but that would be about it.
The manufactures of the solar panels usually have a 25 year workmanship warranty and 25 year production guaranty warranty. Inverter warranties are between 12 to 25 years and some offer extended warranties for purchase.
Finally cleaning solar panels is not very necessary, see the cleaning solar panels blog.