Doesn't the rain clean them?

Rain washes off some loose dust, but it doesn't remove the sticky film of traffic grime, pollen, lichen or bird droppings that builds up over time. As rainwater runs down the panel it also carries dirt to the bottom edge, where it dries in a line along the frame — often exactly where moss starts to grow.

How dirt affects output

Solar cells can only convert the light that reaches them. A general layer of grime reduces output across the whole panel, and heavy patches like bird droppings can shade individual cells, which affects the performance of the panel and sometimes the string it's connected to. The loss creeps up slowly, so many owners only notice when they compare this year's readings with last year's.

How often to clean them

For most homes in the UK, once or twice a year is enough. Panels on low-angled roofs, near trees or farmland, or where birds roost usually need cleaning more often. A good rule of thumb: if you can see dirt from the ground, or your generation has dropped compared with the same month last year, it's time for a clean.

The safe way to clean solar panels

Use purified water and a soft brush. Pure water dries without leaving mineral spots, and a soft brush lifts dirt without scratching the anti-reflective coating. Avoid household detergents, abrasive pads and pressure washers, which can damage the glass coating and seals — and avoid climbing onto the roof yourself. Professional water-fed pole systems clean most roof-mounted panels safely from the ground.