One of the huge advantages of solar PV systems is that they are relatively maintenance-free. There are no moving parts, no plumbing or motors to go wrong or require frequent replacement.
Having said that, the system is still an electrical installation, and so should be tested every five years.
Panels themselves are self-cleaning in normal rainfall (and so long as they are elevated to a pitch of at least 20 degrees). They should still be visually checked, as dirt does eventually build up (especially on the lower edges of the panels), and will eventually affect performance.