Proved the point about washing and polishing the panels at a Stone The Crows get together a few yrs back. People parked near us suffered flat batteries after a few days and both depended on CPAP machines over night as a minimum power requirement. Measure the solar input, then cleaned the panels with truck wash, a soft broom to scrub with and hosed off afterwards. Solar input increased by 50%. Then I polished the panels with No.2 cut and polish and showed all who had gathered to watch just how dirty the polish rag became after the first application on each panel, even though they had been cleaned with truck wash and hosed off. After the job was completed the solar input was now double what it had been before we started, so a 100% improvement. I had a spare solar panel so I set that up as well to assist getting the battery back up to a reasonable state of charge. By lunch time the next day the battery was back in float mode, so I removed my extra panel and for the rest of the week the battery returned to float mode after lunch and they could again use their lights and TV as well as the 2 x CPAP machines.
That seemed like proof enough for me that both cleaning and polishing the panels greatly improves the solar harvest each day. I guess if the solar array is adequately over sized the effect of cleaning and polishing the panels would not be particularly obvious, but the damage to the solar panels caused by that black layer build up that starts from the frame and works its way out is far more obvious, but ya gotta look for that

T1 Terry