As with windows, more light gets through when the glass is clean.
Those of us who live in areas with high pollen count or other atmospheric pollution might want to wash the panels down once or twice a year. Those with panels at a steep angle and a clean environment might never have to think about it. There are probably more systems that don’t get washed than do.
Inverters, disconnect switches, and meters for grid-tied systems require zero maintenance. Battery-based grid-tied and off-grid systems use batteries for energy storage, and these must be maintained according to manufacturer’s instructions.