Rain helps, but in the Central Valley it often comes too late and too light to do the whole job.
What rain can do
In the 2013 UC San Diego study, how much a rain cleaned panels rose with rainfall up to about 0.1 inch, then leveled off. First Solar saw as little as 0.5 mm clean panels fully in areas with light soiling.
So a solid storm can rinse loose dust, especially on steeper roofs.
What rain can't do
NREL found no reliable rain threshold that always cleans panels, and panels didn't always return to full output after rain. Flat or low-tilt panels hold dirt and puddles longer. Bird droppings and sticky road film often stay put.
And in Fresno the rain usually doesn't return until fall, after the panels have been dirty through the sunniest months.
What to do
Look at your panels from the yard or a window. If you see a haze, spots or a dirt line along the bottom edge, rain didn't get it. Text us a photo and we'll tell you if it's worth a wash.