Why Do Solar Panels Need Cleaning (and How Much Output Do You Lose)?

Dirty solar panels typically lose somewhere between 5% and 20% of their output, and in heavily soiled or coastal cases it can be more. Because the loss creeps in gradually, most people never notice — they just quietly generate (and save) less than they should. A proper clean restores that lost output, which is why it usually pays for itself.

Here's what's really going on, and why homes near the coast lose the most.

Why panels lose output when they're dirty

Solar panels make electricity from sunlight hitting the cells. Anything that sits on the glass — dust, pollen, bird droppings, salt, sand, lichen, general grime — blocks some of that light. Less light reaching the cells means less power generated. Simple as that.

Two things make it worse than you'd expect:

• It's cumulative. A thin, even film of grime knocks a few percent off everywhere, all the time.
• Shading hits hard. A concentrated spot — a bird dropping or a patch of lichen — can drag down the output of a whole string of cells, not just the bit it's covering. So even a fairly clean-looking panel with a few stubborn marks can be underperforming.

How much could it be costing you?

If a system loses, say, 10–15% of its generation to dirt, that's 10–15% less self-generated electricity and a smaller feed-in or export benefit, year after year. On a typical domestic array that adds up to a meaningful sum over time — comfortably more than the cost of an occasional clean. The dirtier the panels and the bigger the system, the stronger the case.

Why rain doesn't do the job

People assume rain keeps panels clean. It doesn't, really. Rain runs off and leaves dried mineral deposits behind (the same way it spots a car), and it does nothing for the lower edge of each panel, where grime, moss and lichen collect and bake on. That bottom strip is often the dirtiest part of an otherwise rain-washed panel.

Why coastal West Sussex homes are hit hardest

This is the big one around here. Homes near Chichester Harbour, the Witterings, Bracklesham and Selsey get a constant supply of salt spray and fine sand on the wind. Salt is sticky and hygroscopic — it draws in moisture and forms a haze on the glass that rain alone won't shift, and it encourages green growth around the frames. Coastal arrays genuinely need cleaning more often than inland ones for exactly this reason.

To put rough numbers on it: an inland array in a fairly clean suburban or rural spot typically drifts down by around 5–10% between cleans — a steady film of dust and pollen. A coastal array in the salt-and-sand belt along the West Sussex coast more commonly loses 15–20%, and sometimes more, because the salt haze and baked-on sand build a denser, stickier layer that rain can't rinse away. In other words, the same panels that might lose a tenth of their output a few miles inland can lose closer to a fifth here on the coast — so the case for a regular clean is materially stronger the nearer you are to the water. These are typical guide figures from local jobs, not a lab measurement — every roof, pitch and aspect is different — but the coastal-versus-inland gap is real and consistent.

The safe way to clean solar panels

• Pure (deionised) water and a soft brush, applied with a reach-and-wash pole. Pure water dries spot-free with no detergent residue, and the soft brush won't scratch the glass.
• No harsh chemicals or abrasives — they can damage the anti-reflective coating on the glass.
• No high pressure — a jet can force water past the seals into the panel.
• From the ground where possible, using a pole, so there's no walking on or leaning on the array.

It's closer to careful window cleaning than to pressure washing — gentle, methodical, and all about a clear, residue-free finish.

How often should you clean them?

For most homes, once or twice a year keeps output up. Coastal properties tend to want twice-yearly because of the salt and sand. If you're near trees or have had bird-dropping problems, keep an eye on it more often.

If your generation figures have been quietly drifting down, your panels are probably overdue. We clean domestic and commercial arrays across the coast — see our solar panel cleaning service, or send us your postcode for a free quote.

Need this done properly? See our solar panel cleaning service across Chichester and the harbour villages — or get a free quote.