Solar panel cleaning in California: when it pays, and how to do it safely

What California’s own soiling study found

Felipe Mejia and Jan Kleissl at UC San Diego used California Solar Initiative data from 186 residential and commercial systems, spread from the San Francisco Bay Area to the Mexican border, covering 2010. They compared output right after more than 0.1 inch of rain with output during dry spells. The paper appeared in the journal Solar Energy in 2013. Solar Energy (2013)·checked Sep 23, 2026

On average, panels lost a little under 0.05 percent of efficiency per day without rain. Panels left unwashed through a 145-day summer drought lost 7.4 percent. The university’s release put the payoff plainly: for a typical 5 kW home system, a wash halfway through summer would be worth “a mere $20” of electricity before the drought ended, and “most homeowners won’t get their money back for hiring someone to wash their rooftop panels.” UC San Diego, July 2013·checked Sep 23, 2026

Two details matter for where you live. The researchers found no statistically significant difference between regions during the drought period, although sites in the Los Angeles basin and the Central Valley had dirtier panels. And panels mounted at less than five degrees of tilt lost more, because rain and gravity clear flat glass less well.

When cleaning is worth paying for

The same researchers named the cases where washing makes sense:

• Bird droppings. Heavy droppings block light on individual cells and don’t rinse off like dust. If birds nest under the array, see the bird-proofing guide.
• Very flat panels. Arrays tilted less than five degrees, common on low-slope roofs and carports.
• Heavy local dust. Homes directly beside a highway, factory or agricultural operation.

Ash from a nearby wildfire is not in the study, but it behaves the same way: a visible layer that your output data will show. Whatever the cause, test it with numbers:

1. Compare a dry-season month in your monitoring app with the same month last year.
2. Estimate the kWh a cleaning would recover before the next real rain.
3. Multiply by what that power is worth to you: your rate per kWh if you would have used it, or your export credit if it would have gone to the grid.
4. Compare that with the cleaning quote. If the quote is bigger, wait for rain.

What cleaning quotes depend on, and NREL’s yearly upkeep benchmark, are in solar panel maintenance and cleaning costs.

How often to clean solar panels in California

There is no statewide schedule that fits every roof, and a fixed contract can pay for cleanings the rain would have done. For most homes, the practical pattern is to check output at the end of the dry season, clean only if the loss is worth it, and deal with droppings or ash when they appear. Homes in the dustier cases above may find one cleaning before the rains pays; homes with steeply tilted panels in a clean area may never need one.

What you need to clean solar panels yourself, and how

If your panels are reachable from the ground and your manual allows it, this is what you need:

• A garden hose at normal pressure, or a soft brush or sponge on an extension pole.
• Purified or deionized water if your tap water leaves spots.
• Your panel maker’s manual, open to the cleaning section.
• A cool time of day, such as early morning, so cold water doesn’t hit hot glass.

And what to avoid:

• Walking or kneeling on panels, or working on a roof without fall protection.
• Pressure washers, scrapers, abrasive pads and harsh detergents, unless the manual approves them.
• Spraying water into junction boxes, connectors or under the array.
• Forcing off ice or snow on mountain homes; let it melt.

Your panel maker’s manual is the standard a warranty claim is judged against, so follow its method over any general advice, including this page’s.

Hiring a solar panel cleaner

Ask before anyone gets on the roof:

• What water pressure, water type and tools will you use?
• Can you show a certificate of liability insurance?
• Who pays if a panel, tile or gutter is damaged?
• Will you compare output before and after?

A cleaner should wash, not repair. Anything that touches wiring, connectors, mounts or panel positions is solar work, and the Contractors State License Board’s advice is: “Do not use a contractor who is not licensed to perform solar work.” CSLB·checked Sep 23, 2026 If a cleaner reports broken glass or loose wiring, get it checked by a licensed contractor; our repair cost guide explains what drives that bill.

Cleaning, warranties and leases

Cleaning is maintenance, and the CPUC’s consumer guide says that “unless you purchase a maintenance plan or your system comes with one, you will be responsible for any maintenance and repairs.” CPUC·checked Sep 23, 2026 If a company owns your system under a lease or PPA, check the contract before hiring anyone: it says whether cleaning is your job, and a third party on a leased array can start a dispute.

If cleaning doesn’t bring output back

A loss that stays after a rain or a wash is not dirt. A drop on one panel or one string points to shade, damage or a failed microinverter; a sudden drop across the whole system points to the inverter. Work through the checks for panels that under-produce, and remember that a lower December is normal (see winter production in California). A small, steady decline over years is normal panel degradation, which cleaning can’t reverse.

Planning a new roof? Cleaning is a poor reason to get on the roof, but a reroof is a good time to deal with everything at once; see our roof suitability guide. Panels laid nearly flat on a low-slope roof are the ones that hold dirt longest; the trade-offs are in solar panels on a flat roof.

Cleaning is one part of solar panel maintenance

Washing the glass is the most visible upkeep, but it is rarely the job that protects your production. Monitoring, shade from growing trees, inverter faults, wiring damaged by animals and roof penetrations matter more over a system’s life, and a cleaning company is not licensed to deal with most of them. The full owner’s checklist, who may do each task, the Solar Shade Control Act and a who-to-call table are in solar panel maintenance in California.