Last updated: August 2026
What causes white spots on solar panels after cleaning?
Hard water. San Diego tap runs roughly 250 to 320 ppm total dissolved solids, and when it dries on hot panel glass the calcium and magnesium stay behind as a white film. The spots are mineral deposits, not damage. Rinsing with deionized water at 0 ppm leaves nothing behind to dry.
A Scripps Ranch homeowner called us in June. He had gone up with a hose and a squeegee on a Saturday afternoon, and by Monday every panel had a chalky bloom across the lower third. He thought he had ruined the coating.
He had not. He had just watered his roof with hard water at one in the afternoon.
Why panel glass shows it worse than a window
Panels run hot. Surface temperature on a south facing array on a 78 degree day sits between 130 and 150 degrees, so water flashes off in well under a minute. There is no window of time to wipe it away. The minerals dry in place.
Etching is the part that actually matters. Left through a full summer, deposits bond into the anti reflective coating, and the only thing that lifts them then is an acidic cleaner and a long dwell, which nobody wants to run on a coated panel.
| Tap water rinse | Deionized rinse | |
|---|---|---|
| Dissolved solids | 250 to 320 ppm | 0 ppm |
| What dries on the glass | Calcium and magnesium film | Nothing |
| Squeegee needed | Yes, and fast | No |
| Risk to coating | Etching after months of buildup | None |
How much output do the spots actually cost you?
Soiling loss across San Diego County runs 3 to 7 percent in a normal year, higher on the coast where salt film bonds instead of blowing off.
We metered a 24 panel array in Rancho Bernardo last September at 7.9 percent below its clean baseline after a dry summer. Most of that was not dust. It was hard water haze on the lower third of every panel, put there by a sprinkler head aimed slightly high on the south side of the house.
Sprinkler overspray is the quiet one. It runs at four in the morning, dries by nine, and repeats every other day for years.
What we run instead
Deionized water through a resin tank, water fed pole, soft bristle head, no soap. Zero ppm going on means zero ppm left when it dries, so the panels sheet off and air dry clean with nothing touching the coating but bristles.
A 20 panel residential array takes 45 to 60 minutes and runs $150 to $250, which works out to about $6 to $12 per panel. Thirty to forty panel arrays run $250 to $400.
We do not walk on tile. Broken tile costs more to repair than the cleaning is worth, so pole work from a ladder set at the gutter line covers most single story homes and a good share of two story ones.
Coastal arrays need this twice a year. Inland usually once. We laid out the difference in coastal versus inland solar panel cleaning, and the full service is on our solar panel cleaning page. Scripps Ranch specifics live on the Scripps Ranch array page.