Is a wash worth it?
Two numbers from your app or bill, and the answer in dollars.
What is dirty glass costing?
Two numbers: a year of production from your app, and what you pay per kWh.
How this works
- Yearly production in kWh comes from your app’s yearly view; a year of utility bills works too.
- Price per kWh is your bill’s total charges divided by the kWh it bills. It is never stored or sent anywhere.
- The 7% case is the top of the range in an NREL-led mapping study (Micheli and colleagues, 2019), which reported soiling energy losses of up to 7% at U.S. sites.
- The 2% case is our own illustration of a light film, not a measured figure for your roof.
- The result is an estimate for comparison, not a promise of what a wash will return.
How do I read the result?
Compare the dollar figure to the price of a wash. A wash pays for itself only when the yearly loss is larger than what you pay for it. On a typical home array, a few percent is real but small, so the honest answer is often “not yet.”
What is a few percent worth?
Take an 8 kW roof making 11,000 kWh a year. A 2% loss is 220 kWh; 7% is 770 kWh. At an illustrative 15 cents a kWh, that is about $33 or $116 a year, against a wash from $149. Larger arrays, heavy film and expensive power tip the math sooner.
When does the math change?
A few things push a wash over the line.
Heavy, visible film
Droppings, sap or ash you can see from the yard block more than a light dust.
A large array
Forty panels lose twice the kWh of twenty at the same percentage.
Flat or low tilt
Low-tilt glass sheds less in rain, so film builds and stays longer.
Slipping app numbers
Clear-day output drifting down month over month is better evidence than any calculator.
Questions about the numbers
Why does the tool show a range instead of one number?
Because soiling varies by roof, tilt, trees, roads and weather. An NREL-led mapping study found losses up to 7% at U.S. sites, and many roofs sit well below that. Showing 2% and 7% gives you a light case and a heavy case to weigh against the price of a wash.
My result is smaller than the price of a wash. Should I skip it?
Probably, for now. If the yearly loss is less than a wash costs, let rain and time do what they can and check your app again after the next dry spell. We would rather tell you to wait than wash glass that is not costing you much.
Does a wash include anything besides clean glass?
Yes. Every wash includes a glass-level check of each module, with photos of anything cracked, browned, loose or nesting underneath. Some owners book for that look as much as for the water. The calculator only counts the energy, so weigh the check separately.
Can I trust the dollar figure?
Treat it as a guide. It uses the production or bill you enter and a flat loss across the year, while real soiling builds and clears with the weather. Your own app is the better judge: compare clear days before and after rain or a wash.
What if I only know my monthly bill?
Enter the yearly total of your bills and the tool works from that. It is less exact than production in kWh, since your bill nets out what you use and what you send to the grid. Your inverter or monitoring app usually shows yearly production directly.
Want a second opinion?
Send us your result and a screenshot of your app’s monthly view. We reply within one business day.