Sunprint
At a glance
Yes, dirt matters more in California than in rainy states. In federal soiling data, 128 of 137 California solar sites lost more than 1% of their yearly output to dirt, with a median of 3.2%, and Central Valley sites lost a median of 5.5%. On a typical home's savings, that's tens of dollars a year. A wash is worth it when it costs less than the output it wins back, and dust starts settling again as soon as you're done.
Where dirt costs the most
The Central Valley stands out. Its 24 sites, in Kern, Kings, Fresno, Merced, Sacramento, Tehama and Shasta counties, lost a median of 5.5%, and the highest, in Merced County, lost 10%. Sites around Los Angeles, Orange, Ventura and San Diego lost a median of 2.9%, and the Inland Empire and deserts 3.2%.
A caution on the data: these are mostly commercial and utility systems, not homes. The median site was tilted 7.5 degrees, flatter than most house roofs, and 15 were on trackers. Panel makers say steeper panels clean themselves in the rain, but in much of California the rain doesn't come for months: the 15 sites tilted 15 degrees or more still lost a median of 3%.
The dry season does it
Rainy days a month, and how fast dirt builds up
Panel makers count on rain. Canadian Solar says panels tilted at least 15 degrees "generally may not require cleaning (rain will have a self-cleaning effect)," and REC says normal, regular rainfall cleans its panels if they're installed at a sufficient angle. From May through October, Fresno, Bakersfield, Los Angeles and San Diego average about one rainy day a month or less, and essentially none in July and August.
Meanwhile dust keeps landing. At the median California site, dirt cut output by about 0.11% a day while it built up in summer, and 0.08% in January. At the summer rate, a freshly washed panel is 3% down again in about 27 dry days. The first good autumn rain does the washing for free.
What dirt costs you
| Utility | Saved a year, clean | Lost at 1% | Lost at 3.2% | Lost at 5.5% |
|---|---|---|---|---|
| PG&E | $1,120 | $11 | $36 | $62 |
| SCE | $1,099 | $11 | $35 | $60 |
| SDG&E | $1,542 | $15 | $49 | $85 |
3.2% is the statewide median loss and 5.5% the Central Valley median. Yearly savings are the median for homes in our model whose solar covers 85% of their yearly use, at 2026 rates and export credits (110 PG&E and SCE homes, 88 SDG&E homes). The losses scale those savings by the share of output lost, which is close but not exact, because dirt cuts every hour's output a little.
So a typical home that loses the statewide median gives up roughly $35 to $50 a year, and a Central Valley home losing 5.5% about $60 to $85. Compare that with what a cleaning would cost you, and remember the gain fades as dust returns. One well-timed wash at the end of a long dry spell, or after a dust storm, ash fall or a run of bird droppings, gets more back than a wash in winter, when rain is about to do it anyway.
How to clean without voiding the warranty
Do
- Rinse from the ground with a garden hose at normal household pressure.
- Wash in the cool of early morning or evening; working panels run about 20°C (36°F) above the air.
- Wet the dirt first, then use a soft sponge or soft, non-abrasive brush on a pole; REC allows both, and Canadian Solar asks for a non-abrasive sponge.
- Use low-mineral water if you can, and keep it near neutral (Canadian Solar asks for pH 6 to 8).
- Check your own panels' manual first: makers differ on soaps and tools.
Don't
- Don't brush or wipe dust off dry. Canadian Solar says dry dirt must never be scraped or rubbed away, because it scratches the glass, and that dry-cleaning damage voids its warranty.
- No pressure washers, steam cleaners or high-pressure hoses: REC says they invalidate its warranty. Canadian Solar caps water pressure at 40 bar, about 580 psi.
- Don't stand, walk or lean on the panels.
- Don't clean cracked panels or loose wiring: Canadian Solar warns of shock, and says to disconnect the system before cleaning.
Work from the ground. Canadian Solar's manual warns that "the voltage and current present in an array during daylight hours are sufficient to cause a lethal electrical shock," and damage from cleaning the wrong way voids its warranty. Anything you can't reach safely from the ground is a job for someone trained and equipped to work on roofs.
Do solar panel cleaners need a license in California?
California's contractor board doesn't address panel washing directly. Two rules frame it. The C-46 solar contractor license covers anyone who "installs, modifies, maintains, and repairs" solar energy systems. And the licensing law exempts small jobs under $1,000 in total labor and materials that don't need a building permit, but not for someone who advertises as a contractor or has employees doing the work (Business and Professions Code 7048). If a company says it's licensed, you can look up the license number on the CSLB's website; our company check guide shows how.
What a free quote call gets you
- One verified local installer
- The installed price for your roof
- Your payback years from that price
- Free, with no obligation
See what solar could save you
About a minute, and no phone number needed. If the number looks good, a free quote call gets you the exact price.
Common questions
Do solar panels need cleaning in California?
Sometimes. In federal soiling data, 128 of 137 California sites lost more than 1% of their yearly output to dirt, a median of 3.2%, because months go by without rain. Central Valley sites lost the most, a median of 5.5%. Winter rain does much of the cleaning; a wash late in the dry season gets the most back.
How much does dirt cost on a California solar bill?
In our 2026 model, each 1% of output lost costs a typical home about $11 a year at PG&E or SCE and $15 at SDG&E. At the statewide median loss of 3.2%, that's roughly $35 to $50 a year.
When is the best time to clean solar panels in California?
Late in the dry season, after months without rain, or after a dust storm, ash fall or bird droppings. Clean in the cool early morning or evening. A winter wash gains little when rain is due.
Can I use a pressure washer on solar panels?
No. REC says pressure washers, steam cleaners and high-pressure hoses invalidate its warranty, and Canadian Solar caps water pressure at 40 bar, about 580 psi. A garden hose at normal household pressure is fine.
Can I just brush the dust off?
Not dry. Canadian Solar says dirt must never be scraped or rubbed away when dry, because it causes micro-scratches on the glass, and that damage from dry cleaning voids its warranty. Wet it first and use a soft brush or sponge.
Do you need a license to clean solar panels in California?
The contractor board doesn't address washing directly. Its C-46 solar license covers installing, modifying, maintaining and repairing solar systems, and the small-job exemption (under $1,000, no building permit) doesn't apply to anyone who advertises as a contractor or has employees.
Related guides
- California solar calculator
- Solar panel cost in California
- NEM 3.0 and net billing explained
- Check a solar company
- Solar batteries in California
- How long solar takes in California
The soiling data come mostly from commercial and utility systems, which sit flatter than home roofs. Your own loss depends on your tilt, your neighborhood's dust and how much rain you get. Sources: NREL (now NLR), photovoltaic module soiling map and site data · NOAA NCEI U.S. climate normals, 1991 to 2020 · REC, Quick guide to cleaning (PDF) · Canadian Solar installation manual, U.S. edition 3.3, July 2026 (PDF) · CSLB: C-46 Solar Contractor classification · Business and Professions Code 7048. Reviewed October 2026.
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