Living in Florida, we get a lot of rain that does a good job of keeping our solar panels mostly clean. To see if extra scrubbing was needed, I cleaned 1/2 of my solar panels after they had been installed for 11 months using a scrubbing brush and dish detergent.
The panels that I cleaned went up in power production by an average of 0.11 kWh/month. [0.11 kWh * 36 panels * 12 months = 47.52 kWh of extra power over an entire year…assuming the cleaning effect persists after the first month.] So this is a very small amount of power (about $6 worth at 13 cents per kWh.) in return for an hour of scrubbing. [And this assumes that the cleaning benefit lasts for a full year, which may not be the case.]
I would suggest only scrubbing your (Florida) panels every few years unless you notice a drop in performance.
You can download my data and simplistic analysis in the attached open document spreadsheet: SolarPanelCleaningExperiment
Thanks for letting us know about that. I have my installation on a Spanish Tiled slanted roof scheduled for Feb 10, 2020. Twenty-one panels. I was told not to worry about washing at tilted roof. HOWEVER, since you suggest every few (3 or 4?) years, I wonder whom to hire as I am not in posture to climb or work roof being ‘of age’ (71). Further since I am in a townhouse with four units, our community have roofs washed every so often. What do I say to those workers that are washing ALL the roofs when they arrive at my solar paneled roof? ALL LOVE!
If you have people who already clean the roofs, just tell them to not stand on the solar panels. (They probably use a pressure washer on the tiles, which should also be safe on the solar panels as long as they treat them like skylights.)