How Much Production Do Dirty Solar Panels Actually Lose?
Soiling typically costs a commercial solar array a low single-digit percentage of annual production in average conditions, but in high-dust, low-rainfall environments the loss runs substantially higher and can persist for months because there is no rain to reverse it. The U.S. National Renewable Energy Laboratory puts average annual soiling loss across the United States in the low single digits as a percentage, and the figure for a given site depends far more on local conditions than on any national average.
Source for the national figure: the U.S. National Renewable Energy Laboratory soiling research. Our own documented before-and-after results are in the Soiling and Recovery Report.
What drives the number up or down?
Five factors, in rough order of influence:
- Rainfall frequency. Sites with regular rain get a partial natural reset. Sites without one do not.
- Local particulate sources. Agriculture, unpaved roads, construction, industry, and freight corridors all load an array differently and some of that loading bonds to glass rather than sitting on it.
- Array tilt. Lower tilt angles shed less. Flat and low-tilt commercial rooftop arrays hold more material.
- Particle type. Oily combustion particulate and wildfire ash bond. Dry dust partly sheds. The same visible amount of material can represent very different production loss.
- Time since the last wash. Soiling is cumulative through a dry season.
Why national averages mislead on individual sites
A national average blends a rainy site in the Pacific Northwest with an agricultural ground-mount site in the southern San Joaquin Valley. Those two sites have almost nothing in common. Using the average to set a cleaning interval will overservice one and badly underservice the other. The number that matters is the site's own measured soiling rate. Our Kern County page is a worked example of a site where the national average is the wrong number.
How do you measure soiling loss on a real array?
Compare actual production against expected production for the measured irradiance over time. The gap between the two, once equipment faults and shading are excluded, is the soiling signal. The confirmation test is simple and conclusive: if a wash recovers the gap, it was soiling. If it does not, it was not.
Does cleaning solar panels increase output?
Yes, when the panels are soiled. Cleaning does not improve an array beyond its design performance, it recovers production that soiling was suppressing. The size of the recovery is the size of the soiling loss, which is why measuring before and after matters more than cleaning on a calendar.
What does this mean for a commercial operator?
Three practical conclusions:
- Set the interval from site data, not from a default.
- Treat event-driven washes, after ash-fall or heavy wind, as separate from the baseline schedule.
- Measure recovery. A cleaning program that is not measured cannot be optimized and cannot be defended to a finance team.