5 August 2026

How soiling cuts your solar panel yield

Not every layer of dirt weighs the same: why one splat of bird droppings costs more than a whole dusty panel.

At its core a solar panel is a sheet of glass passing light to the cells beneath. Anything sitting on that glass: dust, pollen, salt haze, bird droppings, blocks part of that light, and light that never passes the glass never becomes power. How much it matters depends on the type of soiling and how long it stays: a thin layer of dust makes little difference, a baked-on grime edge or a splat of bird droppings all the more. Soiled panels yield noticeably less, and the loss quietly grows with the layer.

Not all soiling weighs the same. An even film of dust or pollen dims the whole panel slightly: annoying, but predictable. Localised soiling is nastier. The cells in a panel are wired in series, and a cell that gets almost no light under a splat of droppings drags the whole string down: one dirty corner can cost more yield than an entirely dusty panel. If you live near water or in a city with many pigeons or gulls, droppings are the soiling to watch most closely.

The third category is the creeping one: the dirt edge along the frame. At the bottom of the panel, where the last rainwater lingers and dries, a dark edge of algae, dust and moss builds up over the seasons. That edge does not rinse off in a shower: rain carries away loose dust but leaves static grime, pollen, salt haze and droppings in place. Waiting for a wet week to act as a free clean means waiting for something that never arrives.

How fast the layer grows is mostly down to your surroundings. Near mature trees, pollen and leaf haze lay a film over the glass within weeks in spring; at the coast the sea wind carries a salt haze that rain does not dissolve; in cities and near water, pigeons and gulls are the main suppliers. In an open, clean environment everything moves more slowly. The season does the rest: pollen peaks in spring, dry summer weeks stack up dust, and the breeding season leaves its marks on exactly the panels that shelter a nest.

If bird droppings keep turning out to be the main culprit, fight the cause rather than the symptom every few months: bird proofing closes off the space under the panels with mesh or strips and costs € 195 to € 450 for an average home, including nest removal and cleaning of the panels. That is more money than a single visit, but it structurally removes the fastest-returning polluter.

To find out what soiling is doing in your case, start with your monitoring app: compare the same month with a year earlier. If yield lags without a clear reason, a yield check settles it: the professional inspects the panels, reads out the inverter and tells you whether it is soiling or the system. Such an inspection costs € 85 to € 165 and many companies offset it against a follow-up job.

Finally, make it a habit to open the app at every turn of the month and set the monthly total next to last year's. It takes a minute, and it is the cheapest monitoring there is: soiling never announces itself with an error message, but it does show as a curve that lags a fraction further month after month. Spot that pattern early and you clean in time, instead of running a whole summer season at half strength.

If dirt turns out to be the culprit, the fix is straightforward: a cleaning visit with osmosis water costs € 4 to € 8 per panel, or € 75 to € 175 for an average home. Set that against what your system produces in the sunniest months and the sum is quickly made: clean glass pays for itself. The full rates, including subscriptions and bird proofing, are in the price index.