Bay Area panels get dirty on a schedule of their own: pollen in spring, dust through a long dry summer, ash in fire season, then winter rain that rinses some of it off and leaves the rest.
Late spring, once the pollen stops
Spring tree pollen is sticky, and rain doesn’t fully clear it. When NREL researchers tracked five solar plants in North Carolina through pollen season, rain washed some of the pollen away, yet output never climbed back to where it was before the pollen arrived.
The spring cleaning should wait until the pollen has finished falling, usually May or June. Clean in March and the next wave lands on fresh glass. Wait until August and the pollen film sits on the panels through the months with the longest days and almost no rain. A late, wet spring can push pollen season back a few weeks, so go by when the yellow dust stops showing up on your car, not by the date.
Late fall, after fire season and before the rains
By October the panels have gone five or six months with little rain. Summer dust, the last of the pollen, bird droppings and any wildfire ash have all had time to settle. The fall cleaning comes once fire season winds down, usually October or November, so a late smoke event doesn’t coat freshly cleaned glass.
Fall is also the last dry stretch before the winter storms, which matters for two reasons. A dry roof is a safer roof to work on. And waiting for rain to do the job leaves the stubborn part behind. The National Renewable Energy Laboratory (now the National Laboratory of the Rockies) notes that rain and wind remove some loose dust, but panels cool overnight and collect dew, and dew can glue dust down hard enough that a heavy storm won’t shift it. Droppings are worse: UC San Diego engineers found they shut out almost all the light and survive the rain.
Is winter or midsummer a bad time to clean?
Winter is the hardest season to work in. Days are short, storms push dates around, and wet roofs are slick. Loose dust also gets some help from the rain in those months, so a winter cleaning does the least.
A July or August cleaning is fine when the panels need it, and droppings should never wait. As a routine date, though, midsummer falls inside fire season, and one heavy smoke event can put a fresh layer of ash on the glass a week later. We’d book late fall over midsummer for that reason alone.
When to clean outside the two windows
Some problems shouldn’t wait for May or October:
- Bird droppings on the glass, or pigeons nesting under the panels.
- Wildfire ash after a smoke event.
- New dust from construction or demolition next door, or a site right beside a highway, factory or farm field, which the UC San Diego study found can get dirty enough to need cleaning.
- A production drop on clear days that holds for weeks, once your installer has ruled out an equipment fault.
Two visits a year suits most Bay Area homes, though some roofs need more. If your panels have never been cleaned, here’s what builds up on panels nobody cleans.
Best time of day: morning or late afternoon
The best time of day to clean solar panels is morning or late afternoon, when the glass is cool. Midday sun heats panel glass enough that water dries in streaks before it can rinse, and cold water on sun-baked glass is a thermal-shock risk.
Timing a commercial solar cleaning
For a school carport or a warehouse roof, the site’s quiet hours usually matter more than the month. Our commercial crew works evenings, weekends and overnight windows, so parking lots and loading docks stay open during the day. DOE’s solar operations and maintenance resources tie soiling control to a site’s regular maintenance program, and the right commercial cleaning interval depends on how the site is used.
Total Solar Cleaning works across Contra Costa and Alameda counties from Concord. Call (833) 444-4173 or send your address and panel count through the contact form, and we’ll suggest a spring or fall date for your roof.