Does Rain Clean Solar Panels in Boise?

Only a little. Rain rinses loose dust off tilted panels, but it leaves bird droppings, pollen, smoke film and a line of dirt along the bottom edge, and here in the Treasure Valley the least rain falls right when your panels make the most power.

What rain does wash off

Rain water itself is fairly clean. A good, steady rain rinses loose dust and some pollen off tilted panels, and the steeper they sit, the better it works. Flat and low-tilt panels hold on to dirt longer, because the water sits instead of running off.

Plain dust also builds up slowly. In a UC San Diego study of 186 homes and businesses with solar in California, panels that went 145 days of summer drought without rain or cleaning lost 7.4% of their efficiency. On average, panels in the study lost a little under 0.05% a day. So a light film of dust on its own isn’t an emergency.

What rain leaves behind

The trouble is everything that sticks. Rain runs right over these:

Bird droppings. They bake onto the glass in the sun and shade the cells underneath. Even the UC San Diego researchers, who found dust losses small, said panels heavily soiled with bird droppings should be cleaned.
Pollen. In a study published in 2023, the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) looked at five large solar plants in North Carolina. Output fell by as much as 15% in peak pollen season, and rain washed away some of the pollen but not enough. Even after the pollen season ended, the panels didn’t get back to their cleaned levels, despite frequent rains.
Smoke film. Wildfire smoke leaves an oily, sticky film, and water alone doesn’t cut oil.
The dirt line at the bottom edge. A light rain washes dust down the panel, the water pools against the bottom of the frame, and when it dries it leaves a band of mud across the bottom row of cells.
Dust that dew has glued down. Morning dew wets the dust, the sun bakes it, and after a few rounds it can turn into a crust a passing shower won’t move.

Why rain does even less in the Treasure Valley

Summer is our dry season. The long, sunny days when panels make the most power are the same weeks with the least rain, so dirt sits on the glass right through your best months.
Dust that never lets up. Farm ground, gravel roads and all the building going on around the valley put a steady film on every roof. The UC San Diego team noted that sitting right next to and downwind of a highway or a farm field may make enough dirt to be worth cleaning.
Spring pollen and cottonwood fluff. Fluff collects along the panel edges, and pollen is the kind of dirt the NREL study found rain can’t keep up with.
Smoke in some summers. A smoky stretch lands on panels during the driest part of the year, when there’s no rain to even try.
Sprinklers on ground-mounted panels. Sprinkler water isn’t pure. It carries minerals. If your sprinklers reach panels on the ground, they leave white spots that rain won’t touch.

Does snow clean solar panels?

Not really. Snow usually slides off tilted panels as it melts, and it can drag some dust along, but the melt leaves the same dirt line at the bottom edge. Gray inversion weeks add a dull film by spring. Let snow slide off on its own: don’t climb up to clear it, and never scrape panels with a shovel or a metal tool.

How to tell if rain isn’t doing the job

Look after a rain. If you can still see droppings, a haze or a line of dirt along the bottom edge from the yard or an upstairs window, the rain didn’t get it.
Check your monitoring app. Compare a clear, sunny day now with one at the same time last year. If output stays low even after a good rain, dirt is a likely cause. If it stays low after a cleaning too, call your solar installer.
Think about the season. Panels here are usually at their dirtiest in late spring, after pollen, and in late summer, after the dust and smoke.

Can you just hose them off?

Sometimes, yes. Panels on the ground or on a low mount you can reach are easy to rinse yourself. A few rules keep you and the panels safe:

Read your panel maker’s cleaning instructions first. Some warranties spell out what you can and can’t use.
Pick a cool morning or evening. Cold water on hot glass is hard on panels, and midday sun dries hose water into spots.
Soak, don’t scrape. Let water soften bird droppings, then lift them off with a soft brush. No metal scrapers, scrub pads, harsh cleaners or pressure washer.
Mind the minerals. Hose water carries minerals too, so it can leave its own spots. Rinse well, and don’t let it dry in the sun.
Keep water away from the wiring. Spray from above, never up under the panels.
Stay off the roof. Roof panels mean ladder and roof work, and that’s where the real risk is. Never walk, kneel or lean on the panels.

How we clean solar panels

We look your panels over first, then clean them with soft brushes and pure, filtered water at low pressure, and finish with a clear rinse. No harsh chemicals, no pressure washer and nothing that scratches the glass. Because the water has its minerals filtered out, it dries without spots. A typical home system takes about 1 to 2 hours, and we bring our own water.

We’re fully insured, and you can add solar panel cleaning to an outside window cleaning, so the glass and the panels get done in one trip. We clean panels on commercial buildings too, and we serve the whole valley, from Boise and Meridian to Eagle, Nampa and Caldwell.

When to book

Late spring, once the pollen settles, so your panels are clean for the long summer days. Then again in late summer or early fall if dust or smoke has built up. Here’s more on how often to clean solar panels in Boise.

Rain won’t finish the job, and a roof isn’t worth the risk. Get a free quote or call or text (208) 484-5207, Monday through Saturday, 8am to 7pm.