Sometimes, not always. On a steep UK roof, rain washes off most loose dust, so lightly dusty panels lose little. Cleaning matters when there's bird mess, lichen or a dirt line along the bottom edge, or when panels sit at a shallow angle. Your own generation figures are the best way to tell.
Why rain isn't always enough
We get a lot of rain, and it does rinse panels. But rain only moves what's loose. It doesn't shift:
- Bird droppings, which dry hard and can shade part of a cell
- The bottom-edge line, where rainwater carries dirt down, pools against the frame and dries. Over time lichen and moss can start there.
- Greasy traffic film on homes by busy roads
- Salt on homes near the coast and estuaries, which there's plenty of in Essex
Why it matters more on some roofs
Steep panels self-clean best. Panels on a shallow-pitched roof, a flat-roof frame or a garden mount let water sit and dirt settle. Panels near fields get dust at harvest, and panels under trees get sap and leaves.
How to check your own panels
- Look at them from the garden, or zoom in with a phone camera. Bird mess and a dirt line are easy to see.
- Open your inverter or solar app and note the output on a clear, sunny day.
- After cleaning, compare it with a similar clear day at a similar time of year.
If you see little difference, your panels didn't need it, and you know for next time.
Cleaning them safely
Don't climb onto the roof, and don't use a pressure washer, abrasive pads or household cleaners. Most panel makers recommend soft brushes and clean water. Purified water is best because it dries without spots. Our solar panel cleaning is done from the ground with a water-fed pole.
What's the best way to compare before and after?
Pick two bright, clear days as similar as possible, one just before the clean and one just after. Note the total generation from your inverter or app for each day. Cloud, temperature and the time of year all affect output, so a single day isn't proof. Look at a week or two either side if you can.
Why can a small patch of dirt matter?
Because cells are wired in strings. A shaded or covered cell limits what flows through the rest of its string. Bypass diodes help, but the shaded section still contributes little. That's why a bird dropping can matter more than its size. Read more in do bird droppings on solar panels matter.
When is cleaning not the answer?
When output falls suddenly, or one panel underperforms while it looks clean. That points to shading from a new obstruction, a fault in the panel, wiring or inverter, or a setting that's changed. Speak to your installer. Cleaning only fixes dirt.
How often should they be cleaned?
There's no fixed rule. See how often solar panels should be cleaned in the UK for the signs to look for.