The short answer
Solar output drops as soiling builds: dust, pollen film, and bird droppings block light, and studies measure urban soiling losses around 0.05–0.2% of production per day, compounding until rain or cleaning resets it. Rain clears loose debris but not bonded film or droppings. Safe cleaning means soft brushes and pure or deionized water — never pressure washers, abrasives, or harsh chemistry, all of which panel warranties prohibit.
The Physics of the Drop
A solar panel is a light instrument, and anything between the sun and the cells is subtraction: dust film scatters light, pollen bonds into a haze, bird droppings cast hard shadows that don't just cost their own footprint — a shaded cell drags down its whole string, which is why one well-placed dropping can outweigh a month of dust. Soiling research measures the losses in fractions of a percent per day — roughly 0.05–0.2% daily in urban conditions — which sounds small until it compounds: a dry, pollen-heavy Michigan spring can leave an uncleaned array meaningfully down by early summer, and arrays under trees or near gravel roads run worse. Your inverter app tells the story if you compare a cleaned month against a dirty one at similar sun.
What Rain Does and Doesn't Fix
Rain is a decent janitor and a poor cleaner: it rinses loose dust and larger particles like pollen grains, but bonded films, fine particulates, and dried droppings largely stay — and light rain can make things worse, wetting the dust into a residue that dries as haze. Michigan panels also winter under snow, which mostly slides off self-cleaning glass but leaves the spring surface carrying whatever the melt concentrated. The practical rhythm: most residential arrays here do well with an annual or twice-yearly clean, timed after the heavy pollen drop and, ideally, before peak summer production — clean glass earns the most exactly when the sun pays best.
The Safe Method — and the Warranty Line
Panel manufacturers converge on the same care rules: soft brushes or non-abrasive tools, water — ideally pure or deionized so it dries spot-free, exactly the water-fed-pole technology professional window cleaning already runs — mild non-harsh cleaners at most, and no pressure washers, no abrasives, no metal tools, no harsh chemistry. The prohibitions aren't fussy: pressure can crack cells, force water past frame seals, and scratch the anti-reflective coating that the glass's efficiency partly rides on, and warranty language excludes damage from improper cleaning. Two more rules with no exceptions: cool panels only — cold water on hot glass thermal-shocks it — and roof work belongs to people equipped for roofs.
What a Professional Visit Looks Like
Deionized water, soft-brush agitation, spot-free rinse, panels worked cool in the morning, frames and the roof zone around the array left clean — plus the visual check that comes free: cracked glass, failing seals, nesting under the array, and shading from growth that wasn't there at install. Panel cleaning pairs naturally with window service or a roof visit, quoted by array size and access. What it never involves is the pressure equipment cleaning the driveway — different tool, different job, and the warranty agrees.
The Bottom Line
Dirty panels quietly tax every sunny day, rain won't collect the whole debt, and the safe cleaning method is specific enough that most damage comes from enthusiasm rather than neglect. Check your inverter data, clean after pollen season, and keep pressure washers at ground level where they belong.
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