The water matters as much as the brush. Clean a solar panel with tap water and you trade loose dust for a bonded mineral film. That film costs you output every month it stays on the glass.
What tap water leaves behind
East Bay Municipal Utility District water averages around 150 ppm total dissolved solids, which counts as moderately hard. Those solids are mostly calcium, magnesium, and silica. Spray tap water on a warm panel, let it evaporate in the sun, and the water leaves while the minerals stay. They dry into a thin layer of calcium carbonate and silica bonded to the glass.
EBMUD publishes its annual water quality data for the East Bay, and the hardness numbers are public. For local context, Concord's municipal water supply averages over 150 ppm TDS, enough to leave a visible film after each cleaning if tap water is used.
A single rinse does not look like much. The problem is that the film is cumulative. Every tap-water cleaning adds another layer. After a few rounds the glass carries a permanent haze that plain-water rinsing cannot remove.
Why a mineral film costs you money
Solar panels make power by letting light reach the photovoltaic cells under the glass. Anything on the surface that scatters or absorbs light cuts how much gets through. A mineral film reduces light transmittance. Less light in means less power out.
The loss from a light haze is small on any single day, maybe a few percent. Across a full year of peak production, and over the 25-year life of the panels, those percentages turn into real kilowatt-hours and real money on your PG&E bill. We hear the symptom often: homeowners in Walnut Creek who've had panels cleaned with tap water often notice a hazy residue within a few weeks of service, usually after a handyman or window washer did the job with a hose.
The math in plain numbers
A typical East Bay residential system produces 8,000 to 12,000 kWh per year. A mineral haze that cuts output by 3 to 5 percent removes roughly 250 to 600 kWh annually. At current PG&E residential rates, that is money left on the roof every year, and it grows as the film thickens.
What deionized water does differently
Deionized water has had its mineral ions stripped out. Total Solar Cleaning runs water through a filtration and deionization system until it measures under 10 ppm TDS. At that level there is almost nothing dissolved in the water to leave behind.
When deionized water dries on glass, it evaporates clean. No calcium, no silica, no spots. This is the method recommended in the maintenance guidance of SunPower, Tesla, LG, and Panasonic for warranty-safe cleaning. The gap shows up most in the hotter inland valleys: in Pleasanton, where summer heat accelerates evaporation and concentrates minerals, the difference between DI and tap water is especially visible.
Why a garden hose is the wrong tool
A garden hose does two things wrong at once. It uses raw tap water, so it deposits minerals. And water pressure alone cannot dislodge bonded soiling, so it leaves the original grime and adds a fresh mineral layer on top.
Pressure washing is worse. High pressure can force water past panel seals and crack cells, which voids most manufacturer warranties. Deionized water paired with a soft-bristle brush cleans without that risk: the brush lifts the bonded particles, and the pure water carries them away with no residue.
What to ask before you hire anyone
The water question is a fast way to tell a solar cleaning specialist from a generalist. Ask what water they use. If the answer is a hose or tap water, you will pay for a cleaning that leaves a film behind.
A professional cleaning uses deionized water, a soft brush, and no harsh detergents. The panels come back clear and stay clear until the next normal round of dust and pollen.
Call (833) 444-4173 to schedule a deionized-water cleaning or get a quote. You do not need to be home. Leave the gate unlocked and Total Solar Cleaning handles the rest.