Re: the slightly strange shape of the curve. The drop at the beginning could be due to the matching between the panels getting worse. Solar panels in a bank don't like being in different conditions. As a rule of thumb they will all perform at about the same level as the worst in the bank but at the finer levels you could get less straightforward interactions.

The drop off after cleaning could well be because some of the initial boost could come from the water cooling off the panels: solar panels become less efficient at higher temperatures so the drop of after they're all cleaned could be them warming up again after the water has evaporated.