The 8 kW number, with the assumptions showing
A typical Valley rooftop is about 8 kW — often 17 to 22 panels. Left alone, Phoenix dust, pollen, and monsoon film sit on the glass and steal sun before it becomes a kilowatt-hour.
The calculator uses a cautious U.S. yield of 1,400 kWh per kW per year, a 15% soiling loss, and $0.16 per kWh. That is 8 × 1,400 × 15% × $0.16, which is about $269 a year. It is a floor, not a promise. Phoenix sun is stronger than that national average, and a summer on-peak rate is often higher than sixteen cents.
- Light film, about 10%: roughly 1,120 kWh, about $179 a year.
- A normal dusty year, 15%: 1,680 kWh, about $269.
- Heavy dust, birds, or a missed monsoon, 25%: 2,800 kWh, about $448 — and more if your rate is above $0.16.
Why the monitoring app hides it
Soiling is slow. The app files the drift under weather. Compare a clear week right after a wash to a clear week months later. The gap that does not come back after a light rain is the film.
Rain in the Valley does not finish the job. It turns dust into mud. Mud bakes into a crust that shades cells and is harder to rinse than dry dust would have been.
Does a wash pay on a house this size?
A first-time residential clean is $129 for 16 panels or fewer, $149 for 17–22 panels, and $199 for 23–28. An 8 kW array usually lands in that $149 band. One wash against a $269 year of soiling is an easy yes. Four washes on the yearly plan — $49 a month for 17–22 panels — is how you stop the loss from stacking between monsoons.
Use the calculator before you book. If your sliders land under the price of a visit, wait. If they land over it, the kilowatt-hours are already gone and a drone wash is how you get them back without anyone on the roof.
Your numbers
Run it on your roof
This opens on an 8 kW Phoenix home at 15% soiling and $0.16 per kWh — the same math as the article. Change the size, the loss, or the rate, then book if the year costs more than a wash.
Soiling tax
What is dust costing the array?
Soiled modules quietly clip production — often 5–30%, depending on pollen, dust, and agriculture. This is an indicative annual recovery using 1,400 kWh per kW, a conservative U.S. average. Not a guarantee.
Is $269 the most an 8 kW system can lose?+–
No. That is the calculator’s conservative start: 15% soiling at $0.16 per kWh and 1,400 kWh per kW. Phoenix yield and summer rates are often higher. Slide both up and read your own year.
How many panels is 8 kW?+–
Usually 17 to 22 modern panels. That is the $149 first-visit band, or $49 a month on the quarterly plan.
Will one wash recover the whole year?+–
A wash restores the soiling that is on the glass that day. It does not refund the kilowatt-hours you already lost. That is why a quarterly cadence beats one heroic clean after the crust has baked on.
