Do dirty solar panels lose power? When cleaning is worth it

Yes, dirty solar panels do produce less electricity, because anything sitting on the glass blocks some of the sunlight. In the UK, though, the loss on a typical sloped roof is usually small, and cleaning is mainly worth it when panels are shallow-pitched, under trees, marked by birds or showing a clear drop in output.

How much power do dirty panels lose?

It depends on where you live and how well rain rinses your panels. The first Europe-wide assessment of soiling losses, reported by pv magazine, estimated an average annual loss of 0.9% when rain cleans panels fully, ranging from 4.3% in Greece down to 0.2% in Norway. When the researchers assumed rain removed only a small fraction of the dirt, the average loss rose to 5.3%.

The same study found that a Swiss site with more than 1,000 mm of evenly spread rain still recorded soiling losses of up to 10%, so rain alone doesn't guarantee clean panels.

Pollen is a good example. Research from the US National Renewable Energy Laboratory, covered by Power Engineering, found output fell by up to 15% in peak pollen season, and rain didn't fully restore performance afterwards. That was at large solar farms in North Carolina, so treat it as an illustration of what pollen can do rather than a UK figure.

Why most UK roof panels stay fairly clean

The Energy Saving Trust says rain does a good job of keeping roof-mounted panels clean because of the angle they sit at, and that homes in wetter areas need cleaning less often. Its general advice is to clean every 12 to 18 months, with drier, dustier or pollen-heavy spots needing it more often.

What rain struggles with:

• Bird droppings, which dry hard and shade cells in one spot
• Moss and lichen creeping in from the panel edges
• Leaves, sap and pollen from nearby trees
• Dust from farmland, building work or busy roads
• Panels on low-pitched or flat roofs, where water sits instead of running off

In rural spots around Sileby and Syston, farmland dust and pollen can build up through spring and summer. Nearer the centres of Leicester and Loughborough, traffic film and pigeons are more likely to be the problem.

Signs your panels are worth cleaning

You don't need to clean on a fixed schedule if the panels look clear and the system is performing as expected. Cleaning is worth it when:

• Your monitoring app or inverter shows lower output than the same sunny month last year
• You can see bird droppings, moss or a grey film from the ground
• One row of panels looks noticeably dirtier than the rest
• The panels sit near trees, a farmyard or a building site
• Your installer's warranty asks for regular maintenance (the Energy Saving Trust notes some do, so check your documents)

Compare like-for-like months. Winter output is always lower, so a dull December isn't a sign of dirty panels.

How to clean solar panels safely

Most panels are on a roof, so safety comes first. The Energy Saving Trust advises cleaning from the ground where possible, never standing on panels, using a soft brush, and avoiding pressure washers and harsh detergents, which can interfere with seals and wiring. It also recommends turning the system off first and reporting any damaged cabling to your installer.

That's why we clean panels from the ground with a pure-water, water-fed pole system. Our poles reach up to about 20 m, the soft brush lifts dirt without scrubbing hard, and there's no soap left on the glass. Nobody needs to climb onto your roof, and we're fully insured and FWC trained.

Is professional cleaning worth the cost?

For most homes in Leicestershire, a clean once or twice a year is plenty. The Energy Saving Trust puts professional cleaning at roughly £20 to £200, depending on the size of the system and how easy it is to reach.

If you already have your windows cleaned, adding the panels to the same visit is usually the simplest route. Our Plus plan includes solar panels rinsed twice a year alongside your windows, doors and conservatory, from £35 per visit. You can compare every plan in our domestic window cleaning price guide, or book online with a £10 deposit taken off your first invoice.

Got a conservatory as well? Our guide to keeping a conservatory roof clean uses the same pure-water approach.