Guide · Updated August 2026
The short answer is once a year for most homes, twice if you are near the sea, under trees, or somewhere birds gather. The longer answer is more useful, because it explains why the honest figure for your roof might be “less often than someone is trying to sell you”.
This is the objection we hear most, and it is half right. Rain does deal with loose dust, and if dust were the whole problem then British weather would largely solve it for free.
The trouble is that dust is not the expensive part. Rain does very little to bird droppings once they have dried, nothing at all to lichen, nothing to tree resin, and it does not remove the salt film that coastal roofs pick up — it redistributes it and leaves it to dry again. It also tends to wash debris down the panel and lodge it against the bottom frame rail, which is the one place you least want organic matter sitting wet for months.
This is the part that is genuinely counter-intuitive, and it is the reason percentage figures for “dirty panels” are so slippery.
The cells in a panel are wired in series, like old Christmas lights. Current through a series string is limited by its weakest member. An even film of dust dims every cell slightly, and the output drops by roughly what you would expect — a few percent. But an opaque deposit, a dropping or a patch of lichen, does not dim one cell, it blocks it. That one cell now throttles its entire string until a bypass diode switches the affected section out altogether.
So a handful of small, opaque marks covering a trivial fraction of the array surface can cost far more output than a uniform layer of grime across all of it. Controlled tests on deliberately fouled panels have reported losses well into double figures from a small number of droppings. It also means the loss is lumpy and unpredictable, which is why we are wary of quoting a single confident percentage at anyone.
If your array is steeply pitched, inland, with no trees over it and no particular bird problem, an annual clean is genuinely enough, and there are roofs where we would say every eighteen months is defensible. We would rather say that than sell visits nobody needs — a maintenance plan only works as a long relationship if the customer still believes it is worth it in year three.
What we would not skip on any system past about ten years old is the inspection. Clamps work loose, cable clips perish and drop cable onto tiles, isolators corrode, and inverters develop faults that show up in the readings long before anyone notices the bill. That half of the visit is often worth more than the cleaning.
Ground level, soft brush, telescopic pole, plain water, and pick a cool overcast day so nothing dries onto the glass. Do not use detergent, do not use anything abrasive, and do not use a pressure washer — it can drive water past the frame seal and strip the anti-reflective coating, and cleaning damage is not usually a warranty matter.
Please do not go up on the roof. Falls from height remain the largest single cause of workplace deaths in the UK, and a wet pitched roof covered in glass is a genuinely bad place to find out you are less steady than you thought.
Only partly. Rain shifts loose dust reasonably well. It does not remove bird droppings, lichen, tree resin, salt film or the grime that collects along the bottom edge of a panel — which are exactly the deposits that cost the most output.
For most UK homes, once a year is a sensible baseline and twice a year is worth it if you are on the coast, under trees, or somewhere birds congregate. Inland sites with open roofs can often genuinely manage on annual.
Published estimates for UK soiling vary widely, commonly in the low single digits to around 7% a year for general soiling, with higher figures quoted for neglected systems. The more useful point is that the average is not what hurts you: localised opaque deposits cost far more than their size suggests.
You can, at ground level with a soft brush and plain water. We would not recommend going on the roof. Falls from height are the single biggest cause of workplace deaths in the UK, and a wet pitched roof with glass on it is not the place to learn.
It can. Pressure washers can force water past the frame seal and stress the laminate, and abrasive grit driven at speed will scratch the anti-reflective coating. Damage from cleaning is not usually covered by a panel warranty.
Conditions differ enough between Dorset towns that we have written each one up separately: