Is solar panel cleaning worth it in a rainy climate like Belgium?

If you manage a roof with solar panels, sooner or later someone asks whether they need cleaning. In a rainy country like Belgium, rain washes off some of the dirt, though far from all of it. Here is what the studies say, and how to judge it for your own roof.

Do dirty solar panels affect performance in a rainy climate?

Usually only a little, and not equally on every roof. European and Belgian measurements show losses of a few per cent, with wide differences between installations.

A European study led by the German Aerospace Center DLR (Renewable Energy, 2024) puts the average annual soiling loss in Europe at 0.9% if rain washes the panels fully clean each time. If the researchers assume that rain removes only 10% of the dirt, the same study finds the loss rises to 5.3% (DLR). According to the authors, most installations fall somewhere between those two extremes, depending on the surroundings and the type of dirt.

For Belgium itself there are older measurements. The soiling report from the IEA PVPS programme cites a Belgian study in which the power loss was between 3 and 4% over a five-week period (IEA PVPS). The DLR study cites the same Belgian research for a 3 to 4% yield loss at high pollen concentrations. On an average Belgian roof, large yield gains are therefore not to be expected.

Why doesn't rain wash everything off?

Because not all dirt is equally loose. Loose dust goes with a heavy shower, but according to the DLR study, pollen, bird droppings and industrial pollutants tend to stay on the glass until someone cleans the panels.

According to both the DLR study and the IEA PVPS report, soiling also increases as panels lie flatter. The IEA PVPS report describes a rooftop installation in Burgdorf, a rainy Swiss town, with a railway line less than 100 metres away and farms nearby, where rain had been expected to keep the panels clean. After a dry spell a permanent strip of dirt formed along the lower edge of the panels, and according to the report the loss had built up to 10% by spring 1998. The DLR study names brake dust as an example of dirt that rain struggles to remove.

Is solar panel cleaning worth it on your roof?

Mainly where dirt stays put and where a small loss per panel adds up across many panels.

On large commercial roofs, scale plays a part as well. The IEA PVPS report says a cleaning schedule should be defined for each site, and that the simplest rule is to clean once the cost of a clean is lower than the revenue lost to soiling. The DLR study ran that calculation for seven countries where cleaning costs were known: if rain cleans everything, cleaning paid off in only a few of them, and if rain removes little, at most locations. The authors recommend soiling mitigation in most regions, given high electricity prices. For your own roof, then, what matters is knowing what your installation actually loses.

How can you tell whether your panels are dirty enough to clean?

With your production figures and a visual check, ideally both. That shows whether there is a real loss before you bring anyone in.

Your inverter's monitoring platform shows the yield per day or per month. Compare it with the same period in previous years or with parts of the installation facing the same way. Output that gradually falls behind in comparable weather can point to soiling, although a fault or shading can also be the cause. Because the loss varies strongly from year to year at some sites according to the DLR study, it is best to follow the figures over time.

For the visual check, look from the ground, from a taller building or at photos: at the lower edge of the panels, at droppings and at green growth.

What do manufacturers say about cleaning and warranties?

It varies by manufacturer, so the manual for your panels is the reference. The IEA PVPS report points out that manufacturers typically specify a maximum water pressure and that warranties may be voided if it is exceeded.

One example is the cleaning guideline First Solar publishes for its own modules (First Solar). It names reverse osmosis water as giving the best results, limits water pressure at the nozzle to 35 bar and prohibits spraying pressurised water directly at the seals and the junction box. According to the document, failing to follow the guideline may void the warranty. The same guideline also says that for many installations rain should be enough to remove lighter soiling, and that properly timed cleaning can improve yields mainly where soiling is heavy. Other manufacturers set other rules, so check the make and model of your panels and read their instructions. On purified water, see what is pure water cleaning.

How is cleaning done safely?

By spending as little time on the roof as possible. Going on a roof means working at height, and solar panels add an electrical risk.

Under the Belgian rules on temporary work at height, as summarised by the FPS Employment, collective protection takes priority over personal protection, and work at height only goes ahead when the weather does not endanger workers' safety (FPS Employment). The UK Health and Safety Executive advises treating every roof as fragile until a competent person has confirmed it is not, and warns that some rooflights are hard to see in certain light (HSE).

Then there is the electricity. First Solar warns in its guideline that the voltage in an array during daylight is enough to cause a lethal shock, especially with cracked, wet modules, and that only properly trained personnel should clean modules. How often to clean, once it makes sense, is covered in how often should solar panels be cleaned.

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