How Dirty Solar Panels Affect Efficiency

Short answer: moderate soiling costs a typical Chicagoland array 7-15% of its output, and the loss is not spread evenly, a single bird dropping in the wrong place can cost more than a whole season of dust.

A study by the University of California, San Diego measured roughly 7% efficiency loss from moderate soiling. In higher-soiling environments, including the Midwest through pollen season, published estimates run considerably higher.

Soiling Is Not the Same as Degradation

These get conflated constantly, and the difference decides whether cleaning helps you at all.

Degradation is permanent. Panels lose a small, predictable fraction of their rated output every year as the cells age, most manufacturers warrant something in the range of 80-90% of nameplate output still remaining at year 25. Nothing you do with a brush recovers that.

Soiling is temporary and entirely recoverable. It is dust, pollen, droppings, and film sitting on the glass between the sun and the cell. Wash it off and the output comes back the same day.

So if your system is producing less than it used to, the useful question is which one you are looking at. The tell is the shape of the loss. Degradation is a slow, smooth decline over years. Soiling is seasonal and steppy: output sags through April and May, then jumps back the week after a good cleaning or a heavy rain.

How to Check Your Own Loss in 10 Minutes

You do not need us to tell you whether your panels are dirty. Your inverter already knows. Open your monitoring app, Enphase Enlighten, SolarEdge mySolarEdge, SMA Sunny Portal, Tesla, or whatever your installer set up, and do this:

1. Pull up daily production for the last 12 months. Almost every platform has a year view.
2. Compare this May to last May. Same month, roughly the same sun. A double-digit gap that is not explained by weather is a soiling signal.
3. Look for a clear-sky day and check the curve's shape. A clean array traces a smooth arc. A dirty one flattens near midday because the panels are light-starved exactly when they should be peaking.
4. If your system reports per-panel data (Enphase and SolarEdge both do), sort by output. One or two panels far below their neighbours on a sunny day usually means a physical obstruction, a dropping, a leaf mat, a nest, not a failing panel.
5. Note the date you last cleaned. If the answer is "never" and the system is more than two years old, you have found your gap.

That last case is the one we see most often in Chicagoland: a system installed in the 2021-2023 wave, never cleaned, now three or four pollen seasons deep.

What Causes Soiling in Chicagoland

Pollen dominates April through June. Oak, maple, and grass pollen form a sticky layer that adheres to glass and survives rain. This is the single biggest recoverable loss in the Illinois solar year.

Bird droppings are the worst spot soilers, and the reason is worth understanding properly. Panels in a string are wired in series, so current through the whole string is limited by its weakest cell, the same way a kink in one section of hose throttles the entire line. One opaque dropping sitting over a single cell can drag the output of that entire panel, and in some configurations the string, down by 30-40%. A dropping the size of a coin genuinely can cost more than a season of even dust. If birds are nesting under or around your array, bird proofing stops the problem at its source rather than treating it twice a year.

Tree debris, cottonwood fluff, leaf fragments, maple seeds, packs into frame gaps and along the lower edge, where it holds moisture and creates persistent shaded strips.

Humidity and particulate from Chicago-area traffic and industry lay down a fine grime that never announces itself. It builds so gradually that owners tend to accept the lower output as normal.

Winter soot. Through the heating months, combustion particulate settles on the glass and, unlike summer dust, it is greasy. It holds onto the panel through light rain.

Why Rain Does Not Fix It

Rain rinses loose dust. That is genuinely useful, and it is why panels in a wet climate need less attention than panels in a desert.

What rain cannot do is lift pollen film, dried droppings, or greasy particulate, and in Illinois it frequently makes things worse. Spring rain hits a pollen-coated panel, turns the pollen into a paste, and then the sun bakes that paste onto the glass. It is common for an array to read worse two days after a spring storm than it did before.

There is also the runoff line. Water sheets down the panel and evaporates at the bottom edge, leaving everything it was carrying behind as a dark band along the lowest cells. Because those bottom cells are in series with the rest, that band throttles the whole panel.

The Financial Impact

| System Size | Annual Output | 10% Soiling Loss | Annual $ Lost | |---|---|---|---| | 10 panels | $750 | $75 | $75 | | 20 panels | $1,500 | $150 | $150 | | 30 panels | $2,250 | $225 | $225 | | 100 panels (commercial) | $7,500 | $750 | $750 |

These use a round 10% to keep the arithmetic legible. Your real number depends on tree cover, roof pitch, bird activity, and how long it has been since the last cleaning.

A professional cleaning on a 20-panel array runs $200-$300 and recovers most of that year's loss, see the full Chicago pricing breakdown. Note the honest limit: on a small residential system, a cleaning is worth doing but does not print money. On commercial arrays it clearly does, because the loss scales with array size while the per-panel cleaning cost falls.

When Cleaning Will Not Help

Worth saying plainly, because it saves people money:

• Output dropped suddenly and sharply, overnight. That is a hardware or inverter fault, not soiling. Call your installer, not a cleaner.
• One panel reads zero. Also hardware. Dirt reduces output; it does not zero a panel.
• The array is new and under heavy, growing shade. That is a tree-trimming problem.
• A gradual decline of about half a percent a year with no seasonal pattern. That is normal degradation and cleaning will not touch it.

Because pollen film bonds to glass, the method matters as much as the timing, deionized water versus pressure washing covers why the wrong technique costs you the warranty along with the output.