How Often Should You Clean Solar Panels? An Honest Answer for London Roofs

Most domestic solar arrays in London need cleaning once a year, and twice a year if they sit under trees, near a busy road, or anywhere gulls and pigeons like to perch. Rain handles the loose dust. It does not handle the sticky stuff, and the sticky stuff is what costs you output.

That is the short version. The longer version is worth reading if someone has quoted you for a clean and you're trying to work out whether it's a real job or a nice bit of upselling.

What soiling actually costs you

The industry word for the film that builds up on panels is soiling. It covers everything: dust, pollen, diesel particulates, moss spores, lichen and bird mess.

The International Energy Agency's Photovoltaic Power Systems Programme estimates that soiling causes a loss of annual PV energy production of 3 to 5% across the global fleet, which it puts at an economic loss in the order of 3 to 5 billion euros a year. That is an average across every climate, including deserts where the losses are far worse than anything the UK sees.

So be sceptical of anyone quoting you a 30% gain from a clean. On a normal London roof that number is not real. What is real is a few per cent a year quietly disappearing, plus the occasional bigger hit when something genuinely blocks a cell.

The bigger hit is the one worth caring about.

Why bird mess matters more than dust

A thin, even layer of dust across a panel reduces output a little. Annoying, not dramatic.

A dried bird dropping is a different problem. It is opaque, it sticks hard, and it sits over specific cells rather than spreading out. Panels are wired in series strings, so a shaded or blocked cell drags down the cells connected to it, not just itself. That is why a few well-placed droppings can cost you more than a whole panel's worth of general grime.

Gull and pigeon mess is also acidic and it bakes on. Leave it through a hot spell and it stops being a wash-off job and starts being a soak-and-lift job.

If your array has a bird problem, once a year is not enough. We see this constantly on roofs near parks, water, and flat commercial roofs where gulls loaf about.

What rain does and doesn't do

Rain is genuinely useful. It rinses off the loose, dry, powdery layer, which is most of what lands on a panel in a normal month.

What rain cannot do:

• Lift anything greasy. Traffic film from a main road is oily and rain beads over it.
• Shift bird mess once it has dried and bonded.
• Clear the bottom edge. Water runs down the glass and stops at the frame, so silt collects in a dark band along the lowest few centimetres. Over a couple of years that band grows moss.
• Help at all on a low-pitch roof. Below about 10 degrees, water sits rather than runs.

That bottom edge is the thing homeowners never notice from the ground and the thing we see on almost every array that has never been cleaned. It also matters more than its size suggests, for the same series-wiring reason as bird mess.

There is one more catch. Light rain after a dry spell is worse than no rain, because it wets the dust into a paste and then dries it onto the glass. A drizzly week in August will often leave panels dirtier than a dry one.

So how often, really?

Here is what we would tell a neighbour.

Once a year is the sensible baseline for a typical pitched roof in a residential London street, with no trees directly over it. Late spring is the best slot, once the plane tree pollen has finished and before the summer peak generation months.

Twice a year if any of these apply:

• Trees overhang or sit close enough to drop sap, seeds or leaf debris
• The property is on a main road, a bus route, or near a construction site
• You get regular gulls, pigeons or crows on the roof or ridge
• The array is on a flat or very shallow roof
• The panels are on a commercial unit near industrial activity

More often than that is usually not worth paying for on a domestic system. Anyone selling you a quarterly schedule for a nine-panel house array is selling you a schedule, not a benefit.

Check after specific events rather than waiting for the calendar: after building work next door, after a long dry spell that ends in a dust storm off a site, or after a windy autumn if you're under trees.

How to tell if yours need doing

You don't need to get on the roof, and you shouldn't.

Look at your inverter or app history. Compare a clear, sunny day this month against a clear, sunny day at the same point last year. A gradual drift downwards over two or three years, with no other explanation, is usually soiling. Panels do degrade with age, but that is a fraction of a per cent a year, not a visible step.

Then look at the panels from the ground, from an upstairs window of the house opposite if you can, or take a photo with your phone's zoom. You are looking for a dark stripe along the bottom edge, white or grey splashes, green tinges at the frame, or a general dullness compared with the crisp black they had when installed.

If they look clean from below and the numbers look normal, leave them. Not everything needs a service.

Please don't do it yourself with a ladder

This is the part where we'd rather lose the job than have someone hurt.

Cleaning a domestic array means working over a pitched roof, usually two storeys up, often on a wet surface, holding something. The Work at Height Regulations 2005 exist because falls from height remain one of the biggest causes of serious injury in this country, and a domestic roof does not become safer because it's your own.

There are also two ways to wreck the panels themselves:

Pressure washing. A jet wash will drive water past the frame seals and can crack the glass or damage the backsheet. Nobody who cleans panels for a living uses one on them. If a quote mentions jet washing your array, get a different quote.

Tap water and a squeegee. London's mains water is hard, which we cover in more detail in our guide to deep cleaning a bathroom in a hard water area. Let hard water dry on glass in the sun and it leaves a mineral film that is worse than the dirt you removed, and it goes on getting worse each time you do it.

What a proper solar panel clean looks like

The method that works is boring, which is a good sign.

Purified water, fed through a telescopic pole with a soft brush head, worked from the ground. The water is run through a deionising resin so there are no dissolved minerals left in it. That matters because it means the glass dries with nothing on it, no towels, no squeegee, no streaks. It is the same principle as modern professional window cleaners use on upper storeys.

The brush agitates, the water floods and rinses, and the panel is left to dry clear. Stubborn bird mess gets a dwell time rather than more force. No detergents, no abrasives, nothing that leaves a residue for the next rain shower to smear about.

A decent operator will also look while they're up there. Cracked glass, a lifted frame, a chewed cable, a slipped tile under the mounting. You want that reported, not cleaned over.

Our own solar panel cleaning service works exactly this way, reach-and-wash from the ground wherever access allows, which is the great majority of London houses. Where access genuinely doesn't allow it, we say so rather than improvising.

When it isn't worth it

A few honest cases where we'd tell you to save your money.

If the array was installed less than a year ago and there are no trees or birds, wait. If your generation figures are flat against last year and the glass looks fine, wait. If the panels are shaded by something structural, a chimney, a dormer, next door's extension, cleaning will not fix that and no cleaner should imply otherwise.

And if what you actually want is the whole exterior sorted, panels included, it is cheaper to do it in one visit than three. Most people who book us for panels are also thinking about the render or cladding and the windows, and it's the same access equipment.

FAQ

Does cleaning solar panels really increase output? It restores output that soiling was quietly taking away, which is not the same as making the panels better than new. On a lightly soiled array the difference is small. On one with bird mess, moss along the bottom edge, or years of traffic film, it is obvious in the numbers. The honest answer is that it depends entirely on how dirty they were, and anyone promising a fixed percentage before seeing the roof is guessing.

Can I clean solar panels with washing up liquid? No. Detergent leaves a film that attracts dust and can streak badly in sun. Purified water and a soft brush is the method, with no chemicals at all in the great majority of cases.

Will cleaning scratch the glass? Not with soft brushes and flowing water, which is what the glass is designed for. Scratching happens when someone uses an abrasive pad, a scraper, or brushes a dry panel and drags grit across it.

Is there a best time of year to clean solar panels in London? Late spring, once tree pollen and blossom have finished, so the panels are clear going into the highest generation months. Autumn is the second choice if you're under trees.

Does cleaning affect my panel warranty? Using a pressure washer or abrasives can void it. Purified water and a soft brush is the manufacturer-accepted approach on effectively every domestic panel we've come across, but check your own installer's documentation if you're unsure.

Do solar panels need cleaning if it rains a lot? Rain deals with loose dust and nothing else. In a wet year you will still get bird mess, sap, traffic film and a silt line along the bottom edge, and heavy rain does not remove any of them.

Getting it looked at

If you want a straight answer on whether your array actually needs doing, send us a photo from the ground and we'll tell you honestly. If it needs a clean we'll quote it, and if it doesn't we'll say so.

Get a free, no-obligation quote, call us on 07375 104665, or message us on WhatsApp at wa.me/447375104665 for a same-day answer. We cover North London and the Hertfordshire borders.

Last updated: 21 August 2026

About Techno Clean: a London deep-cleaning company covering North London and the Hertfordshire borders. We specialise in one-off deep cleans, end of tenancy and after builders cleaning, fully insured with £5 million public liability cover and DBS-checked teams, rated 4.9 from over 180 reviews across 500+ completed jobs.