How Much Power Do Dirty Solar Panels Lose?
It depends on where you live and what’s on the glass. Field studies have measured losses from a few percent a year to more than 20% in dry, dusty deserts.
The short answer: a range, not one number
How much power dirt costs depends on four things: how much dust lands, how often it rains, how steep the panels sit and what the dirt is. Here’s what field studies have found, from mild to extreme:
- Up to about 7% a year in parts of the U.S. The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL, renamed the National Laboratory of the Rockies in 2025) reported in 2021 that the energy lost to dirt each year runs as much as 7% in parts of the United States, and as high as 50% in the Middle East.
- About 7% after one dry summer. UC San Diego engineers looked at 186 homes and businesses with solar in California. Panels that went through a 145-day summer drought with no rain and no cleaning lost 7.4% of their efficiency.
- Up to 15% in pollen season. NREL found that output at five solar plants in North Carolina fell by as much as 15% in peak pollen season, even though it rains there often.
- 3% to 39% over a year in a desert. In Chile’s Atacama Desert, panels left uncleaned for 12 months lost 3% or less of their yearly energy at higher sites and in the south, and up to 39% on the northern coast, where heavy dust meets very little rain.
- 18% to 40% in one to three weeks. In Al Seeb, Oman, test panels left uncleaned for one, two and three weeks made 18%, 33% and 40% less power.
So the range runs from a few percent to well over 20% in dry, dusty deserts.
Why the losses are so different from place to place
- How much dust lands. The UC San Diego team said panels right next to and downwind of a highway, factory or farm field may get dirty enough to need cleaning.
- How often it rains, and how hard. A good rain rinses loose dust off tilted panels, but a light one can make it worse: in Cyprus, researchers found that dust blown in from the Sahara usually ends with a light rain that sticks it to the panels. More in does rain clean solar panels.
- How steep the panels sit. Flat and low-tilt panels hold on to dirt longer, because water and dust don’t run off as easily.
- What the dirt is. Loose dust dims the whole panel a little. Pollen, smoke film and bird droppings stick, and rain doesn’t move them much.
Why bird droppings cost more than their size
Dust is a thin film. A bird dropping is a solid patch: the UC San Diego engineers said droppings block nearly all the light under them and won’t wash away in the rain, and that panels heavily soiled with them should be cleaned. And the cells in a panel are wired together in a chain, so shade over part of one cell can pull down more of that panel’s output than the spot’s size suggests.
What this means for your roof in the Treasure Valley
We couldn’t find a published soiling study for Boise, so we won’t make up a number. Here’s what we do know about panels here:
- Summers are dry and dusty. Farm ground, gravel roads and all the building around the valley put a steady film on every roof, right through the long days when panels make the most power.
- Spring brings pollen and cottonwood fluff, which stick to the glass and collect along the panel edges.
- Some summers bring wildfire smoke, which leaves a sticky film rain doesn’t rinse off.
- Bird droppings bake onto the glass, shade the cells underneath and stay put through the rain.
Where your roof lands in that range depends on your own spot. A home by the fields in Kuna or out toward Lake Lowell in Nampa, or next to new building in Star or Middleton, collects dust faster than a home on a quiet, finished street.
Winter is a poor time to judge. Short days, snow on the panels and gray inversion weeks all cut output, clean panels or not.
How to tell what dirt is costing you
Your own monitoring app beats any study. Use the clear-day check in how often to clean solar panels in Boise, and keep these in mind:
- Skip smoky and hazy days. Smoke in the air cuts the sunlight reaching your panels, so those days read low even on clean glass.
- If output is still low after a cleaning, call your solar installer. Dirt isn’t the only thing that lowers output.
Will cleaning bring all that power back?
Cleaning can only win back what the dirt was costing. We won’t promise a percentage, because it depends on how dirty your panels are. For plain, light dust, the UC San Diego engineers found the loss often wasn’t big enough to pay for a cleaning on its own. Panels with droppings, heavy dust or a smoke film have more to gain.
Can you clean them yourself?
Panels on the ground are easy to look after. Check your panel maker’s cleaning instructions, then use a soft brush and plain water on a cool morning or evening: no pressure washer, scrub pads or harsh cleaners. Most Treasure Valley water is hard, so don’t let it dry in the sun, or it can leave mineral spots. More on cleaning solar panels with tap water.
Panels on the roof 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 clean panels with soft brushes and pure, filtered water at low pressure. No harsh chemicals, no pressure washer and nothing that scratches the glass, and because the water has its minerals filtered out, it dries without spots. We’re fully insured, and you can add solar panel cleaning to your outside window cleaning so everything gets done in one trip.
We clean panels on homes and commercial buildings across the valley, from Boise, Meridian and Eagle to Caldwell and Emmett. The same dust is hard on windows, too, as one customer in the Eagle foothills found:
“I highly recommend Double Take Window Cleaning. My windows look so clear! I live in the Eagle foothills and we have wind and dust and it’s so hard to make our windows shine, but Double Take surely did. It was a great price for a truly excellent job. Thank you Double Take!!!”
Rather stay off the roof? Get a free quote or call or text (208) 484-5207, Monday through Saturday, 8am to 7pm.