Ask most Naperville commercial solar owners how often their array gets cleaned, and the answer is some version of “the rain takes care of it.” It’s a reasonable assumption, until you look at what the research actually says. Rainfall helps, but rarely all the way: a season’s worth of pollen, road dust, exhaust, and bird droppings layers up faster than summer storms can rinse it off, and bird droppings essentially never wash off on their own. By the time July’s peak production hits, the typical commercial array in the Chicago area is quietly running a few percentage points below where it should be, and on heavier-soiling sites the gap is bigger.
A few percent doesn’t sound like much. On a commercial system, over a Chicago summer, it adds up to real money. Here’s what the numbers actually look like, why summer is when the loss hurts most, and how DuPage County owners can think about getting it back.
The Honest Answer: A Few Percent, Sometimes More
The peer-reviewed research on soiling is consistent on the broad shape: typical U.S. commercial systems lose around 5% of output to soiling, with the realistic range running from roughly 2% to 7% in most U.S. conditions and climbing higher at sites with heavy local soiling factors. The high end of the global range goes as far as 25% or more, but those numbers come from arid, low-rainfall regions like the Middle East and aren’t representative of Chicago. The honest answer for a typical Naperville commercial array sits in the low-to-mid single digits across the year, with peaks during long dry stretches between summer rain events.
That sounds modest. It’s not, once you do the math on a real system.
Why Summer Is the Worst Time to Lose Output
Soiling losses are a percentage of whatever your panels would otherwise be generating, which means the dollar cost depends entirely on when the loss happens. Lose 5% in January, when daylight is short and output is naturally low, and the absolute hit is small. Lose 5% in June, July, and August, when your array is producing the most kilowatt-hours it’ll generate all year, and the same percentage drains far more revenue. Summer is when commercial arrays earn most of their annual income. It’s also, unfortunately, when soiling builds fastest.
What Makes a Chicago-Area Summer So Hard on Solar Panels
Three things converge on a Naperville commercial array between June and September.
Pollen, dust, and exhaust
DuPage County’s mature tree canopy unloads a heavy wave of pollen in late spring that carries into early summer. Add road dust from the I-88 and I-355 corridors, agricultural drift on systems near open land, and exhaust from any of the area’s active construction sites, and the panel surface is collecting fine particulate matter constantly. These thin films are what summer rain partially rinses off, but rarely completely.
Bird droppings: the loss rain can’t fix
Bird droppings are the single most damaging soiling type. They sit on the glass as hard, opaque spots that block individual cells entirely, and once an individual cell is shaded, modern panel architectures can throttle the output of the whole string it’s part of. Rain doesn’t remove them, sun bakes them on, and the longer they sit the harder they are to clean off without damaging the panel surface.
Heat compounds the problem
Solar panels lose efficiency as they heat up. Hot, sunny Chicago summer days drive panel surface temperatures well above ambient air temperature, and the resulting efficiency drop happens on top of whatever the array is losing to soiling. You can’t fix the heat. You can fix the dirt.
Doing the Math: What Soiling Costs a Commercial Array
Take a representative example: a 250 kW commercial rooftop array in DuPage County. A system like that might produce somewhere on the order of 300,000 to 350,000 kWh in a year, with a meaningful share of that generation falling between May and September. Apply a 5% soiling loss across the year and you’re looking at roughly 15,000 to 17,500 kWh of lost production. At commercial electricity rates and current incentive structures, that loss translates into thousands of dollars of unrealized value annually, and far more on a system with heavier local soiling factors or one that hasn’t been cleaned in years. The capital was already spent on the array. Soiling is the only major loss driver an owner can recover, simply by cleaning.
The Good News: Most of It Is Recoverable
Soiling losses are almost entirely reversible. A proper cleaning restores the panel surface to near-baseline performance, and a sensible cleaning cadence keeps output close to optimal year-round. For most Naperville commercial sites, one professional cleaning in spring after pollen settles and a second later in summer for heavy-soiling sites is enough to make a real difference. Touchless drone cleaning makes the operational side painless: no roof access, no foot traffic on the panels, and the work is typically done in a fraction of the time a manual crew would need.
If you’d like a sense of what soiling might be costing your specific Naperville or DuPage County array, Spotless Drone Cleaning would be glad to take a look at the site and put rough numbers behind a cleaning plan. No pressure, just a clearer view of what’s recoverable.
