Dirty panels cost some owners a few dollars a year and others a fair amount more. You can work out which group you are in with your app, your bill and a calculator.
How much can dirt take?
The best public number we know comes from an NREL-led mapping study by Micheli and colleagues (2019). It reported soiling energy losses of up to 7% at U.S. sites.
That is a ceiling, not an average. Many sites in the study lost less, and the loss depended heavily on local dust, rain and how the panels were mounted.
So we use a range. On our output check, the low end is 2% and the high end is the 7% from the study. Your roof sits somewhere in that band, or below it, and the rest of this article is about finding where.
Why is there no single number?
Because soiling is not one thing and it does not stay still. A thin dust film costs less than a dropping that covers a full cell. Pollen in June costs more than the same pollen in December, when the days are short.
It also builds over time. A wash resets the glass, and then the film creeps back through the seasons. The loss you feel is the average over that whole cycle, not the worst week.
And it depends on place. A steep roof in open sky in Wheat Ridge does not collect what a low carport beside a gravel lane in the Arvada farmland does.
How do you work out your own figure?
It takes four numbers and about twenty minutes.
- Your year’s production: the total kilowatt-hours from your monitoring app for the last twelve months.
- What a kilowatt-hour is worth to you: the rate or credit on your utility bill.
- A loss range: try 2% and 7%, the band from the NREL study.
- The cost of a wash: from $149 for up to 20 panels, $5 a panel over that.
Multiply your production by each percentage. Multiply the result by your rate. Compare the two answers with the wash price.
If your utility credits exported energy at a different rate than it charges for energy you buy, use the lower one to be cautious. Your bill shows both, and your utility can explain how your plan works.
Can you show the arithmetic?
Here it is with round numbers chosen for easy math. They are not a forecast for your roof.
- Production: say your app shows 9,000 kWh for the past year.
- Value: say each kilowatt-hour is worth 14 cents to you.
- At 2%: 180 kWh, worth about $25 a year.
- At 7%: 630 kWh, worth about $88 a year.
On those numbers, a single $149 wash does not pay for itself in energy within a year, even at the top of the range. That is a real result, and we would rather you hear it from us.
Double the production, or double the rate, and the picture changes. A larger array, a dirtier site or a pricier kilowatt-hour can move a wash from “nice to have” to “clearly worth it.” Run your own figures before you decide.
Does the season change the math?
Yes, and it is worth thinking about before you book. A film costs the most when the sun is doing the most. On the Front Range, that is the long, high-sun stretch from May to August.
Unluckily, that is also when pollen, cottonwood fluff and summer dust arrive. A June film sits on the glass through the most productive weeks of the year. The same film in December, under a low sun and short days, costs far less in kilowatt-hours.
That is why the timing of a wash matters as much as the frequency. A wash in late June clears the spring film before July. A wash in September clears the summer dust before it bakes on through autumn. A wash in January rarely earns back much, unless something heavy is sitting on the glass. The Front Range solar year lays out the months side by side.
So is a wash ever worth it on energy alone?
Sometimes, and you will know when. It tends to be worth it when one or more of these is true:
- A big array: 30 or more panels, so the same percentage is more kilowatt-hours.
- A dirty site: gravel roads, fields, roosting birds or heavy tree drop.
- A low tilt: flat roofs and carports that rain does not rinse.
- A visible problem: droppings or sap sitting across full cells, not just a light film.
Droppings deserve a special mention. A bird dropping that shades part of a cell does more than its size suggests, because cells are wired in series and the shaded one can hold back its neighbors. One small shadow can cost a whole panel more than a thin film across the array.
What does dirt cost that is not energy?
Some costs do not show on the bill:
- Missed damage: a cracked corner or lifted sealant nobody saw for two years.
- Nests underneath: pigeons settle in under dirty, quiet arrays and bring droppings and debris with them.
- Warranty risk from the wrong wash: a pressure washer or soap can do more harm than the dirt did.
- Second-guessing: not knowing whether a low month was dirt, weather or a fault.
This is where a wash earns its keep beyond the kilowatt-hours. Every visit includes a glass-level check of each module, photographed. If something needs your installer or a roofer, you hear about it while it is small. If you would rather have the look without the wash, a visual solar inspection is from $129.
What does maintenance cost in general?
For context, NREL’s 2020 PV operations and maintenance cost benchmark put residential O&M at $28.94 per kWdc per year, and commercial rooftop at $18.55. Those totals include management and other items, not only cleaning.
To use it, multiply by your system size in kilowatts. A 7 kW home system, again as round arithmetic, comes to a little over $200 a year by that benchmark. That is a useful sanity check on any maintenance plan, ours included.
Our two-season plan is from $259 a year for two washes with a glass-level check each time. Whether that is good value depends entirely on your numbers, which is why we ask for them first.
When should you skip the wash?
When the arithmetic says so and nothing is visibly wrong. That covers more roofs than a cleaning company usually admits:
- Steep roofs in open sky: rain and snowmelt do part of the work.
- New installs: give them a full season before deciding anything.
- Small arrays with light film: the dollars rarely add up.
- Winter months: low output means low loss, so a wash can often wait for spring.
If you are in one of these groups, a one-time wash every year or two may be all you need. Or none, with a look from the ground each spring.
How do you check the loss is real?
Before and after. The monitoring desk pulls the last 30 days from your app the day before we arrive, so we have a baseline. After the wash, we compare the next clear days with that baseline and send you both, along with photos.
You can do a version of this yourself. Note a few clear-day totals before a wash and a few after, in the same few weeks so the season barely moves. The difference is your soiling loss, measured on your own roof.
We do not promise a percentage, because nobody honest can. We show you the numbers and let them speak.
Where to go from here
Put your year’s production and your rate into the output check and see where the 2% to 7% range lands for you. If the figure is small, wait and look again in spring. If it is not, you can book a wash and see the difference in your own numbers.