Hard Water Spots on Solar Panels in Las Vegas: How to Remove and Prevent Them
Short answer: Las Vegas tap water averages 280 ppm hardness, or 16 grains per gallon, according to the Las Vegas Valley Water District, and when it dries on warm glass it leaves white mineral spots. Fresh spots usually come off with deionized water, a soft brush and patience, never acid or a razor. The fix that lasts is simple: stop rinsing solar panels with tap water.
Why Las Vegas water leaves spots on solar panels
About 90 percent of Clark County’s water comes from Lake Mead, fed by the Colorado River. The Las Vegas Valley Water District’s 2026 water quality summary lists average total hardness at 280 ppm (16 grains per gallon), total dissolved solids at 591 ppm, calcium at 75 ppm and magnesium at 24 ppm.
Water evaporates; minerals do not. On a sun-warmed panel, a hose rinse can dry in minutes and leave every one of those minerals behind as a white film or a pattern of dots.
- Hosing panels with tap water, especially in the afternoon
- Sprinkler overspray on ground mounts and patio-cover arrays
- Cleaning companies that rinse with plain tap water
- Pressure washer mist, which also puts the panel warranty at risk
What panel makers say about cleaning water
| Source | What it says about water | What it means in Las Vegas |
|---|---|---|
| First Solar cleaning guidelines | Tap water is fine below 75 mg/L total hardness. From 75 to 180 mg/L it must be squeegeed off to prevent scale, and that hard water is prohibited on anti-reflective coated modules. | Las Vegas averages 280 mg/L, well above First Solar’s hard water band |
| LONGi O&M manual | Accepts municipal water, but warns that minerals deposit on the glass after long-term use and can reduce light transmission and output. No acid or alkali. | Repeated tap-water rinses add up over time |
| REC quick guide to cleaning | Deionized water is the ideal choice; tap water is a fallback. Acid or alkali detergents must not be used. | Deionized water is the panel maker’s first choice |
| UNLV (Boeing) study | Distilled water gave the best cleaning results in Las Vegas testing. | Local research points the same way |
None of these makers say a few spots will ruin a panel. They do agree that mineral-heavy water is the wrong thing to keep putting on the glass, and that harsh chemicals are worse.
How to remove hard water spots from solar panels
This is how we take spots off, and it is also the safe way to try it yourself on panels you can reach from the ground.
- Start when the glass is cool. Early morning in summer. LONGi says the water and panel temperatures should be within 10°C, and REC says to clean only cool panels to avoid thermal shock.
- Soak with deionized water. Let it sit for a minute to soften the deposits before you touch the glass.
- Use a soft brush or microfiber with light pressure. Repeat the soak instead of scrubbing harder.
- For stubborn spots, REC allows isopropyl alcohol diluted below 10%. Rinse the area afterward.
- Finish with a deionized rinse. It dries clear, so you do not trade old spots for new ones.
- Check from the ground. Our photo report shows before and after, and we tell you if any marks did not fully come off.
What not to use on hard water spots
- Vinegar, descalers and acid cleaners. REC and LONGi both rule out acid and alkali products on panel glass.
- Razor blades, scrapers or scouring pads. LONGi warns against blades and anything that scratches the surface.
- Pressure washers. REC says pressure and steam cleaners are not permitted and will invalidate the warranty.
- Hot water. Uneven heating and cooling can crack panels, according to LONGi.
Can hard water spots damage solar panels?
Not overnight. A light film of spots blocks a little light, and LONGi’s manual says minerals left on the glass over the long term can reduce transmittance and power output. The bigger risk is usually the removal: acid cleaners, blades and abrasive pads can damage the anti-reflective coating that helps panels catch light.
If your output looks low, compare a clear day this month with the same month last year in your monitoring app. Our is solar panel cleaning worth it guide walks through the math, and residential solar panel cleaning covers what a full visit includes. For other water and cleaner questions, see our FAQ.
How to prevent hard water spots
| Water | Hardness | On solar panels |
|---|---|---|
| Las Vegas tap water | About 280 ppm (16 gpg) | Leaves white spots as it dries |
| Softened tap water | Hardness minerals swapped for sodium | Can still leave a film; not the same as deionized |
| Deionized, distilled or RO water | Close to zero | Dries clear; the panel makers’ preferred choice |
- Stop hosing panels with tap water. If you clean them yourself, a soft brush and a final rinse with deionized or distilled water avoids the problem.
- Aim sprinklers away from ground mounts and patio-cover arrays.
- Ask any cleaner what water they rinse with. It is the first of our 8 questions to ask any solar panel cleaner.
- Clean at dawn, not in the afternoon, so water does not flash-dry on hot glass.
- Keep runoff on the roof. Southern Nevada treats water running off a property as water waste, with fines from $40 up to $5,000 for repeat violations.
Hard water spots on solar panels FAQs
Why do my solar panels have white spots after cleaning?
Las Vegas tap water is very hard, about 280 ppm according to the Las Vegas Valley Water District. When it dries on the glass, the minerals stay behind as white spots. A final rinse with deionized water prevents it.
Can I use vinegar to remove hard water spots from solar panels?
We do not recommend it. Vinegar is an acid, and panel makers including REC and LONGi say acid or alkali products should not be used on the glass. Deionized water, a soft brush and patience are safer.
Will rain wash hard water spots off my solar panels?
Rain is low in minerals, but Las Vegas averages only about 4 inches a year, and UNLV found it took at least 0.2 inches of rain to even partly restore dusty panels. Most spots stay until the glass is cleaned.
Is deionized water safe for solar panels?
Yes. REC names deionized water as the ideal choice for cleaning its panels, and First Solar prefers purified water over hard tap water.
Can hard water spots become permanent?
Deposits left for years can be harder to remove, and harsh removal methods can damage the coating. We use soft methods only and tell you in the photo report if any marks remain after cleaning.
Get rid of hard water spots
Tell us your ZIP code, panel count, roof type and a photo of the spots if you can get one from the ground. We will reply with a price for a deionized cleaning. See typical solar panel cleaning prices.
Sources: LVVWD 2026 water quality summary, First Solar module cleaning guidelines, LONGi O&M manual, REC cleaning guide, UNLV (Boeing) water quality study, KTNV hard water, UNLV rainfall study, SNWA water waste. Updated October 2026.