You see the white spots from the driveway. They are mineral film, and they lift with deionized water and a soft-bristle brush on cool glass.
What are hard water spots on solar panels?
The spots are dissolved minerals, not dirt. Tap water carries calcium, magnesium, and silica. East Bay municipal water averages around 150 parts per million of dissolved minerals, a figure East Bay Municipal Utility District publishes in its annual water quality data. When that water sits on warm glass and dries, the water leaves and the minerals stay. They dry into a thin film that sticks to the surface.
Three things put that film on a panel. A prior clean done with tap water or a garden hose leaves a fresh film behind, because the water that rinses the panel also carries the minerals that dry back onto the glass. Lawn sprinkler overspray does the same when it sprays onto the panel and dries in the sun. And hard water that dries after rain or a do-it-yourself hose rinse leaves the same spots.
How do you remove them safely?
The safe way to remove the film is deionized water and a soft-bristle brush, worked on cool glass. The brush lifts the bonded minerals off the surface. The deionized water, held under 10 parts per million of dissolved minerals, carries them away and dries clean. That is the same method that prevents the film in the first place, and our post on deionized water versus tap water explains the chemistry.
The safe method comes with two more rules. Clean in the morning or late afternoon, when the glass is cool, so the water does not flash off before it rinses. And keep everything that touches the glass soft. The panel's anti-reflective coating is the surface you are trying not to touch.
Why do the usual hard-water fixes make it worse?
The advice that strips a shower door is the wrong advice for a solar panel. A razor blade or an abrasive pad scratches the anti-reflective coating, and that scratch is permanent. A pressure washer forces water past the panel seals, the same failure mode that voids a warranty. Our post on soft washing versus pressure washing covers that risk.
Acids and household descalers are built for bare glass and porcelain. A solar panel's glass carries a coating, and no manufacturer cleaning guidance calls for acid. The guidance is water and a soft tool, nothing else.
When will the spots not come off?
There is an honest ceiling to removal. A surface mineral film lifts with the right method. Scale that has sat through a full dry summer bonds harder and takes more work, and some of it may stay. If the coating has been scratched or etched by a prior bad clean or a razor blade, that damage is in the glass itself, and no cleaning puts it back.
You can usually tell which case you have before anyone climbs up. Spots that appeared recently, after one sprinkler season or one bad rinse, tend to lift. A haze across the whole panel, or spots that have been there for years through repeated dry summers, are more likely to have damaged the surface. We will tell you which one you have, and we will not sell you a cleaning that cannot fix it.
What this looks like in Concord, Danville, and Pleasanton
Concord's hard municipal water makes tap-water mineral film the most common staining source we see on East Bay panels. Danville's landscaped lots put sprinkler overspray onto panel glass, where it dries into white mineral spots. Pleasanton's hot summers dry sprinkler and tap water fast, which concentrates the minerals and makes the spots harder to remove.
Total Solar Cleaning removes mineral film with deionized water and soft-bristle brushes, no acids, no abrasives, no pressure washing. The crew is ISCA certified and carries $2 million in liability coverage. Call (833) 444-4173 for a quote, or request one online. You do not need to be home for the work itself.