How much output do dirty solar panels lose?

Light dust costs a few percent; heavy dust, smog films or bird droppings can cost 15–25% or more. In dusty seasons, monthly losses are commonly in the double digits before cleaning.

How often should solar panels be cleaned?

In dusty or smoggy regions, every 2–4 weeks during dry season; in rainy or clean climates, a few times a year is often enough — let your production numbers tell you.

Cleaning Solar Panels: How Much Dirt Really Costs You

Applies worldwide · Updated July 2026 · 4 minute read

Soiling is the quietest thief in solar. Nothing breaks and no alarm sounds — your system just produces less every day until someone climbs up with water. Here is what dirt actually costs and how to clean without causing damage.

What the losses look like

A light dust film scatters a few percent of light. A dry-season crust, winter smog layer, or a patch of bird droppings is far worse — losses of 15–25% are routine, and a dropping shading one cell can drag down its whole string. If your monthly generation drifts down with no weather change, soiling is the first suspect.

Cleaning safely

Clean at dawn or after sunset — cold water on hot glass can crack it and voids warranties.
Use plain water and a soft brush or sponge on a pole; never detergents, abrasives or pressure washers near the frame seals.
Hard water leaves mineral films; if your supply is very hard, a final rinse with filtered water keeps the glass clear.
Never stand on panels, and treat sloped rooftop work as the fall hazard it is.

How often, by climate

Dusty plains and construction areas: every 2–4 weeks in the dry season. Coastal sites: monthly, with attention to salt film. Smog belts: more often in winter, when the film builds fastest. Rainy temperate climates: rain does most of the work — a few cleanings a year suffice. Panels tilted 10° or more self-clean noticeably better in rain than near-flat panels.

Let the numbers decide

Note your best clean-panel month as a baseline. When production drops several percent below it in similar weather, that is your cleaning signal — measured, not guessed.

Suspect dirt is already costing you? Measure the gap with the system health check, or get your clean baseline from the system size calculator.