You will probably never have to clean them. They should self-clean in the rain. The exception to this is:
a. If your panels are flat.
b. You live in a very dry area.
c. You have a really stubbornly soiled panel due to moss, dead animals, particularly sticky bird poo etc.
If your solar production does drop – then there may be something on the panels that requires cleaning. The best way to find out is a visual inspection.
Safety warning! If you don’t feel confident on the roof and poking around near electrical wiring, get a professional; solar systems produce deadly voltages.