How often should solar panels be cleaned?

Rules of thumb for cleaning solar panels range from once a year to never, and none of them fits every roof. Here is what the interval depends on, what manufacturers advise and how to decide using your own data.

How often to clean solar panels: is there a fixed rule?

No. The right interval varies with the roof and its surroundings, and sometimes from one year to the next.

The soiling report from IEA PVPS, the photovoltaic programme of the International Energy Agency, calls cleaning schedules by climate zone a useful guideline, but says the schedule should be redefined for each PV site (IEA PVPS). A European study led by DLR (Renewable Energy, 2024) points to large year-to-year variation and calls for continuous monitoring. The DLR study puts the average annual soiling loss in Europe at 0.9% if rain is counted as perfect cleaning, and up to 5.3% if rain removes only 10% of the dirt (DLR). Whether that justifies a clean is covered in is solar panel cleaning worth it?.

What role does the tilt angle play?

A large one. The flatter the panels, the less the rain rinses them.

According to the IEA PVPS report, soiling losses typically decrease as the tilt angle increases, and the DLR study also links lower tilt angles with higher losses. Trina Solar's manual for its Vertex modules says that panels installed horizontally, at 0°, should be cleaned more often, because they lack the "self-cleaning" effect of panels at 10° or more (Trina Solar). On a flat or low-pitch commercial roof, rainwater also sits on the glass until it evaporates, leaving its dirt behind.

Does the type of panel matter?

Yes, especially the lower edge. On framed panels, a band of dirt that rain no longer removes can form just above the frame.

The IEA PVPS report describes an installation in Burgdorf, Switzerland, tilted at 30° in a region with rain spread over the whole year. After a dry spell, a permanent band of dirt still formed along the lower edge of the frame. Because the cells were only 1 to 2 mm from the frame, the band hit output hard, and a periodic cleaning schedule was introduced. The same report notes that frameless modules may make soiling easier to remove.

Which surroundings make solar panels dirty faster?

Anything that puts dust, soot or organic matter on the panels. Farmland, industry, a railway, trees and birds are the best-known sources.

The IEA PVPS report names industrial pollution, road traffic, construction sites and intensive agriculture as sources of airborne dust. According to the DLR study, pollen, bird droppings and industrial pollutants are likely to stay stuck to the glass until a proper clean, and rain may remove only a small share of train brake dust, which adheres strongly.

Does the season make a difference?

Yes. Spring brings pollen, autumn brings leaves, and a dry summer can bake dirt onto the glass.

In Burgdorf, pollen from the surrounding forests and farms visibly soiled the panels every spring, and the stubborn band formed after a dry, sunny spell from late July to early November. The IEA PVPS report also shows an example calculation for one site in which the optimal cleaning interval shifts over the year. Snow is a separate case: according to REC, clearing it is not needed for good output, because it usually falls when irradiation is at its lowest (REC).

What do manufacturers say about cleaning?

Clean gently, with pure water and without high pressure or abrasives. Departing from the manual puts the warranty at risk.

REC recommends cleaning the panels when dirt can be seen on the glass, ideally with deionised water, using a soft sponge, a microfibre cloth or a soft, non-abrasive brush. High-pressure hoses, pressure and steam cleaners, knives and metal sponges are not permitted by REC and invalidate the warranty. Trina bans steam cleaners, requires that cleaning tools do not wear the glass and recommends soft water. The IEA PVPS report adds that manufacturers typically specify a maximum pressure.

Hard tap water leaves white marks on glass as it dries. Why water without minerals avoids that is explained in what is pure water.

How do you know your solar panels need cleaning?

By tracking your production figures and having the panels looked at regularly.

These signs point to a clean:

• output drops gradually compared with the same period last year, and does not recover after rain;
• two comparable strings or inverters on the same roof drift apart;
• you can see a band of dirt along the lower edge, bird droppings, leaves or green growth.

A visual inspection shows where the dirt is. A thermal inspection goes further: according to the IEA PVPS report, uneven soiling can cause hotspots that permanently damage cells, and a thermal camera makes them visible. More on that in drone inspection of buildings.

Can you clean solar panels on the roof yourself?

On the roof, we advise against it, for safety reasons.

The UK Health and Safety Executive (HSE) states that all work on roofs is highly dangerous, even if a job only takes a few minutes, and that almost one in five deaths in construction work involve roof work, many of them among people simply repairing or cleaning roofs (HSE). REC asks for fall protection when working at height and forbids standing, walking or leaning on the panels. Trina points out that cracked panels carry an electric shock risk, which is higher when they are wet. If you can reach the panels from the ground with a soft brush on a pole, follow the manufacturer's manual.

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