There's no fixed rule. Many UK panels on a steep roof only need an occasional clean, because rain rinses off loose dust. Panels on shallow roofs, near fields, roads, the coast or trees, or with bird droppings on them, may need cleaning once or twice a year. Your own generation figures tell you best.
Does rain clean solar panels?
Partly. Rain rinses loose dust and pollen off panels on a reasonably steep roof, which is why many UK systems stay fairly clean on their own. What rain doesn't shift well is bird droppings, sticky road film, salt, and the line of dirt that builds up along the bottom edge where water runs off and dries.
So the honest answer depends on where your panels are and what lands on them.
Which panels need cleaning more often?
Panels in dirtier spots, and panels that can't rinse themselves. In Essex, that usually means one of these:
- Shallow roofs and flat-roof frames. Water runs off slowly, so dirt settles and stays.
- Homes near fields. Dust at harvest and ploughing time can coat panels quickly.
- Homes near busy roads. Diesel film is sticky and builds up over time.
- Homes near the coast or estuary. Salt dries into a film that rain doesn't rinse.
- Homes under trees or with birds about. Sap, leaves and droppings sit on the glass.
How do you know if your panels need cleaning?
Look at them and look at your figures. From the garden, check for droppings, a dirty bottom edge, green or grey patches and leaves. Then compare your inverter or app readings on bright days this year with similar days last year.
A gradual drop, with visible dirt, points to cleaning. A sudden drop, or one panel underperforming, may point to a fault, shading or a problem with the system, which is one for your installer, not a cleaner.
How much difference does cleaning make?
It varies too much to give a reliable number. Lightly dusty panels on a steep roof may show little change. Panels with droppings, lichen along the edge or a thick film can show a clearer improvement. Our post on whether cleaning solar panels makes a difference covers how to check it yourself.
We'd always rather you check your own figures than take anyone's word for it.
What's the best time of year to clean them?
Late spring is a sensible time for many homes. Panels make most of their power in spring and summer, so having them clean before the long days makes the most of it. Cleaning after the pollen and blossom have settled means it lasts. If you're near fields, after harvest is another good time.
How should solar panels be cleaned?
Gently, with soft brushes and clean water. Most panel makers' care guides warn against pressure washers, abrasive pads and harsh chemicals. Purified water is ideal, because it dries without leaving spots or a residue, so no detergent is needed.
Don't walk on panels and don't climb onto the roof to reach them. A water-fed pole lets them be cleaned from the ground on most homes, including many two-storey roofs.
Can I clean them myself?
On a bungalow or a ground-mounted array, often yes, with a soft brush on a pole and a hose with no nozzle. Avoid washing-up liquid with tap water if you can, as it can leave a film. Never clean panels from a ladder resting on the panels themselves, and never stand on the roof.
For higher roofs, our solar panel cleaning uses pure water and a soft brush from the ground, as a one-off visit with no contract.