Most Front Range rooftops do well with one or two washes a year. Your trees, your road and your roof pitch decide which, and your own production data can confirm it.

So how often, really?

For a typical home array in the Denver metro, plan on a wash once or twice a year. One after the spring pollen and cottonwood fluff settle, and one after the dry, dusty end of summer.

Some roofs can stretch to a wash every year or two. A few need more than two. The difference is rarely the panels themselves. It is what grows, drives and roosts around them.

There is no fixed schedule that suits every roof. A crew that sells you a monthly plan without looking at your numbers is guessing, and you are paying for the guess.

What does dirt actually take?

Less than you might fear, and more than nothing. An NREL-led mapping study by Micheli and colleagues (2019) reported soiling energy losses of up to 7% at U.S. sites.

Two parts of that sentence matter. “Up to” means many sites lose less. “Sites” means the loss depends on where the array sits: a steep roof in open sky and a low roof beside a gravel lane are not the same case.

That range is why every visit starts with your data rather than a pitch. If your roof sits at the low end, a wash can wait. The output check shows what a 2% to 7% loss is worth on your array, and what dirty panels actually cost walks through the arithmetic.

What lands on panels here?

The Front Range year delivers its own mix, in a fairly reliable order:

  • April to June: tree pollen, a yellow-green film that sticks to morning dew.
  • Late May into June: cottonwood fluff and seeds, thickest near creeks and older tree-lined streets.
  • May to August: hail, which rarely dirties glass but sometimes damages it.
  • July to October: dust and road grime, plus ash in smoky summers.
  • All year: bird droppings, which block light in one spot and shrug off rain.
  • November to April: snow, which mostly slides off by itself.

The Front Range solar year lays this out month by month, with what each season does to the glass and to output.

Why two washes, and why then?

Because the film builds in two waves. Pollen and cottonwood fluff stick to dew-damp glass through May and June. By late June the trees have mostly finished, so a wash then holds through the longest, strongest days of the year.

The second wave is dust. July to September is usually dry along the Front Range, and the afternoon storms that do arrive often drop too little water to rinse anything. Light rain can even glue dust in place, which we cover in does rain clean solar panels?

A September wash clears that late-summer film while the sun is still high enough to matter. Our two-season plan is built around those two windows. It is where many rooftop owners land, but it is not the only sensible schedule.

Who needs more than two?

A few arrays collect film faster than the seasons alone explain:

  • Gravel road roofs: passing traffic lifts fine dust that settles every dry week.
  • Acreage and farm roofs: tilled fields, hay and livestock dust land all summer.
  • Low-tilt arrays: flat commercial roofs and carports shed less in rain, so dirt stays.
  • Pigeon roofs: droppings build up where birds roost on the frames and ridge.
  • Heavy tree cover: foothill pines drop needles and pitch; cottonwoods drop sap.

For these, a third wash or a short midsummer spot visit can be worth it. Your app will show whether it is, and we will tell you if it is not.

Who can wash less often?

Plenty of owners. If your array sits on a steep south-facing roof with open sky, no trees overhead and no dirt road nearby, the heavier storms and snowmelt may keep it in fair shape for a year or more.

A newly installed array also starts clean. Give it a full spring and summer before you decide anything.

If you have a single-story roof or a ground mount, you may be able to rinse it yourself between visits. Our guide to cleaning panels yourself, safely covers when that is fine and when it is not.

How can you tell from the ground?

Start with light, not a ladder. On a clear morning, stand where the low sun glances off the panels.

  • A dull, even haze: pollen or dust film across the whole array.
  • Dark dots or streaks: droppings, sap or leaf litter, usually in clusters.
  • A brown line at the bottom edge: rain carried dirt down and left it at the frame.
  • One row duller than the rest: often the row nearest a tree, vent or roost.

Binoculars help. So does comparing the panels to a clean window on the same side of the house.

What do your numbers say?

Then check the data, because your eyes can be fooled by angle and glare. Pick a few cloudless days this month and compare them to cloudless days from the same month last year, or to the weeks right after your last wash.

A slow slide on clear days, with no new shade and no change in season, points to soiling. A sudden drop points to something else: a fault, a snow slab, a storm. Our guide to reading your solar app shows where to find the daily and monthly views that make this comparison easy.

When we come out, we do this reading for you. The day before a wash, the monitoring desk pulls the last 30 days from your app. After the wash, we compare the next clear days against that baseline and send you both.

Does Colorado sun change anything?

A little. Sun along the Front Range is strong and the air is dry, so glass runs hot by midday in summer. Pollen and dust that land on dew in the morning dry into a crust by noon, and that crust does not lift in a passing shower.

In winter the sun sits low and light strikes the glass at a shallow angle, so any film left on it matters more. That is one reason a September wash still earns its place going into the short days, even though summer is over.

It also shapes when we work. We wash at air temperature, early in the morning or late in the afternoon, and never on hot glass in full midday sun.

Does washing affect your warranty?

It can, if it is done wrong. Panel makers set out how their glass should be cleaned, usually water at air temperature and soft tools, and some tie the warranty to following the installation manual.

That rules out pressure washers, soap and anyone standing on the modules. Our method stays inside those limits, and why we never pressure wash solar panels explains the reasoning in full.

What does a wash cost?

A single wash is from $149 for up to 20 panels, plus $5 for each panel over 20. The two-season plan is from $259 a year for two washes. Steep or two-story roofs can add $25 to $50.

Prices are indicative and confirmed from a photo or satellite view of your roof. Every wash includes a glass-level check, where the crew looks at each module for cracks, browning, loose clips and nesting underneath. The panel cleaning page shows the whole visit, start to finish.

Where should you start?

With your own numbers. Run them through the output check, or email us a screenshot from your app and we will tell you plainly whether a wash can wait until spring. If it cannot, you can book a morning that suits you.