Rain keeps solar panels clean. Output degrades over time naturally regarldess.

I lived off grid with a 2kW system for a decade,. As a test we cleaned themmonce at about the 5 yr mark with soap, water and a broom. My measuring equipment wasn't sensitive enough to notice any difference.

That said I have seen small robotic cleaners (like robotic window cleaners) used on vast solar arrays, possibly an array that vast and a small increase is worth it?