The short answer: dust costs less than you think, and cleaning services cost more than they are worth to most homeowners. But there are three situations where the calculation flips completely.
What soiling actually costs
In most of the United States, accumulated dust and pollen reduce annual production by roughly 2–5%, and rain removes most of it several times a year without intervention. On a system producing 14,000 kWh a year and offsetting power worth 14 cents, a 3% loss is about $59 a year. A cleaning service charging $250 is not a good investment in that scenario.
In the desert Southwest the picture is different but not as different as cleaning companies suggest. Phoenix and Las Vegas see longer dry spells, so soiling can reach 6–9% by late spring in a dry year. That is worth roughly $120–$180 annually on a typical system, which makes one cleaning a year approximately break-even and two clearly not worth it.
The three situations where cleaning genuinely pays
1. Bird droppings and nesting debris
This is not uniform soiling. A single opaque dropping on a cell can force a bypass diode to shunt an entire third of a panel, and with a string inverter that can drag down neighbouring modules too. Localised, opaque soiling is disproportionately expensive. This is also why we push critter guard so hard in Phoenix, Tucson and Las Vegas, pigeons under an array are a production problem as well as a hygiene one.
2. Agricultural, industrial or construction dust
Sticky soiling from nearby fields, cement plants or a year-long construction site does not wash off in rain. We have measured 15%+ losses on systems near active agricultural operations. Those systems need scheduled cleaning and it pays for itself several times over.
3. Low-tilt and flat-roof arrays
A panel at 5 degrees of tilt does not self-clean. Water sheets across it rather than running off, leaving a residue line, and dust accumulates at the lower edge in a band that never clears. Commercial flat-roof arrays almost always need scheduled cleaning; a 25-degree residential roof usually does not.
How to clean panels without damaging them
If you are going to do it yourself, from the ground where possible:
- Cool glass only. Early morning or evening. Cold water on a 160°F module can crack it.
- Deionised or soft water if you can. Hard water leaves mineral deposits that are worse than the dust.
- Soft brush or a dedicated solar squeegee. No abrasives, no scouring pads, nothing that scratches the anti-reflective coating.
- No pressure washers. They drive water past the frame seals and void most module warranties.
- Never walk on modules. Ever. Microcracks from foot traffic are invisible and permanent.
Honestly, for most people the right answer is a garden hose from the ground twice a year in dusty months and nothing else.
What matters far more than cleaning
Monitoring. A single failed optimiser or microinverter costs more production in a month than dust costs in a year, and it is invisible from the ground. We have found systems running 22% down for over a year because nobody was watching the app.
If you are going to spend money on system care, spend it on knowing when something breaks, not on washing glass that rain would have handled.