Why dirty panels make less power
Solar panels need clear glass to let sunlight through. A light layer of dust makes a small difference, but heavier grime, lichen at the edges and especially bird droppings can shade parts of a panel. Because panels are usually wired together, a patch of shade on one can drag down the output of others.
Doesn’t the rain clean them?
Partly. Rain helps with loose dust, but it won’t remove baked-on droppings or the grimy film that builds up, and panels on low-angle roofs don’t drain well. Homes under trees, near farms or near the bay tend to get dirtier faster.
How to tell if yours need a clean
Check your inverter display or monitoring app. If output on clear, sunny days has dropped compared with the same time last year, dirty panels could be part of the reason. You may also be able to see droppings or grime from the ground or a window.
How to clean them safely
Always follow your panel manufacturer’s cleaning guidance. Use soft brushes and clean water, never harsh scrubbing, abrasive cleaners or high pressure on the glass. And please don’t climb on the roof yourself — it’s the most dangerous part of the job. Once or twice a year is enough for most homes, and it’s easy to do at the same time as a roof clean.