Vicaya GlobalCalculator · Solar · cleaning
Is it worth cleaning your solar panels?
Dirt costs output and cleaning costs money, and the right schedule is whichever makes those two together smallest. For most domestic arrays in a rainy climate, that is no schedule.
What do kWp and kWh mean?
- kWh — kilowatt-hour
- A unit of energy, and the thing your electricity bill charges you for. A 1,000-watt heater running for one hour uses one kWh. A typical home uses somewhere between 2,000 and 10,000 kWh a year depending on where it is and how it heats.
- kWp — kilowatt-peak
- A unit of capacity — how big the solar array is, not how much it makes. It is the output the panels would produce under standard test conditions: bright, cold and perfectly aimed. Real roofs rarely see those conditions, which is why a 4 kWp array does not generate 4 kW for most of the day. One modern panel is roughly 0.4 kWp, so 4 kWp is about ten panels.
- Putting them together
- kWp is the size of the system; kWh is what it produces over time. The ratio between them — kWh generated per kWp installed, per year — is the honest way to compare locations, because it strips out how big the system happens to be. It runs from roughly 700 in cloudy high latitudes to over 1,800 in sunny deserts.
12 × 450 W at 21% is 2.14 m² per panel — about 26 m² of roof.
Leave it to the rain. Cleaning this array costs more than the dirt does — never cleaning loses £2 a year, and the cheapest schedule that improves on that does not exist.
Rain is the reason. It rinses the array roughly every 8 days here, which resets the dirt before it has time to build up — and a clean booked for a week it has already rained on buys nothing at all. The drier the site, the more that changes.
Every schedule, costed both ways
| How often | Cleans a year | Average soiling | Energy lost | Worth | Cleaning cost | Total a year | Against never |
|---|---|---|---|---|---|---|---|
| Never — let the rain do it — best | 0 | 0.17% | 10 kWh | £2 | £0 | £2 | +£0 |
| Twice a year | 2 | 0.17% | 10 kWh | £2 | £240 | £242 | −£240 |
| Three times a year | 3 | 0.17% | 10 kWh | £2 | £360 | £362 | −£360 |
| Quarterly | 4 | 0.17% | 9 kWh | £2 | £480 | £482 | −£480 |
| Every two months | 6 | 0.17% | 9 kWh | £2 | £720 | £722 | −£720 |
| Monthly | 12 | 0.16% | 9 kWh | £2 | £1,440 | £1,442 | −£1,440 |
| Fortnightly | 26 | 0.14% | 8 kWh | £2 | £3,120 | £3,122 | −£3,120 |
| Weekly | 52 | 0.10% | 6 kWh | £1 | £6,240 | £6,241 | −£6,239 |
Notice what happens at the frequent end. Cleaning weekly does keep the glass cleanest — the average soiling figure keeps falling all the way down the table — and it is still the worst option on the page, because each visit costs more than the dirt it removes was ever going to take. A cleaning schedule optimises money, not cleanliness, and those have different answers.
How the soiling is modelled
Dirt accumulates at the rate you set, day by day, until it reaches the ceiling you set — glass does not keep getting dirtier for ever, and a tilted panel sheds more than a flat one. Two things reset it: rain, on its own cycle, and cleaning, on yours. Soiling on any given day is measured from whichever happened most recently.
The loss is then weighted by that day's own generation. This matters more than it sounds. A 5.40 kWp array in London, United Kingdom makes several times as much in June as in December, so dirt that accumulates through a bright dry spell costs far more than the same dirt in midwinter. Multiplying an annual average soiling figure by an annual generation total — the usual shortcut — misses that entirely.
What this does not model
- The soiling rate is your figure, not a prediction. Nothing here knows about the pollen, the construction site, the farm track or the gulls near your roof. Published soiling rates span more than a factor of ten between sites, and the named options above place familiar conditions on that range rather than measuring anyone's.
- Rain is an average interval, not weather. Real rain arrives in clusters and dry spells, and a dry spell in high summer costs disproportionately; an average smooths exactly the variation that matters most. Every figure here is averaged across all the ways the rain could line up with your cleaning dates, because otherwise the answer depends on whether the two intervals happen to share a factor — which is arithmetic, not weather.
- Light rain can make soiling worse. Drizzle on a dusty panel leaves streaks and deposits. Only rain heavy enough to rinse is modelled, and only as a full reset.
- No snow, and no bird droppings. Snow is a total loss while it lies and then cleans the glass as it goes; localised soiling on one cell can cost far more than its area through the module's bypass diodes. Neither is a uniform derate and neither is here.
- No damage risk, and no cost of access. Walking a roof has a cost that is not the cleaner's invoice.
Frequently asked questions
Do solar panels need cleaning?
Sometimes — but on a tilted roof, rain has often already done the job. This calculator weighs the generation that dirt actually costs against what a cleaner charges, so you can see whether cleaning pays for itself in your climate rather than cleaning on a schedule.
How much does dirt reduce solar output?
It depends on rainfall, the panel tilt, and how much dust or pollen your area throws up — a dry, dusty site loses far more than a rainy one. The tool models the soiling loss and how often rain resets it, so the figure fits your conditions.
How often should solar panels be cleaned?
Only as often as the lost generation is worth more than the cleaning — which in a rainy climate may be rarely or never. Enter your figures and the calculator tells you whether, and how often, cleaning is worth paying for.
Does rain clean solar panels?
Largely, on a tilted roof — which is why the calculator factors rainfall in before deciding whether paid cleaning is worthwhile at all.