The short answer: pitch decides whether rain is enough

Most UK homeowners are told at some point that solar panels are self-cleaning. That claim is not wrong — it is just incomplete. It is true of a steeply pitched roof, and it becomes progressively less true as the angle flattens out.

Think of the two arrays in the episode. On the left, a steep main roof: during a downpour, water gathers speed as it runs, and moving water has energy to spare. It lifts loose dust, pollen and light soot and takes them with it. On the right, a near-flat garage roof: the same rain arrives, but it has nowhere to go in a hurry. It sits in shallow puddles, and the only way it leaves is by evaporating. Everything it was carrying stays exactly where it was.

So the rule of thumb is simple. The steeper your array, the more likely rain alone will keep it in reasonable condition. The flatter it is, the more likely it will repay an occasional wash. Nothing about the panels themselves has changed — only the geometry.

Why sheeting water cleans and standing water doesn't

The distinction that matters is between water that leaves and water that dries. Sheeting water is a transport mechanism: it flows off the module, and whatever it has picked up flows off with it. Standing water is a settling tank: the water goes, the solids stay.

You can see the evidence on any near-flat array after a dry week following rain. The puddle edges show as pale rings or tide marks, because that is where the suspended material was concentrated as the last of the water evaporated. Add a British spring's worth of pollen bloom, a summer of dry dust and the fine grey film that road and chimney traffic deposits everywhere, and you get a layer that builds up slightly more with each cycle instead of resetting after each shower.

There is a second, sneakier version of the same problem even on pitched roofs: the bottom edge of the module. Most frames sit slightly proud of the glass, so a thin lip of water and silt can be held along the lowest inch of the panel. That narrow band is often the dirtiest part of an otherwise clean array, and it matters more than its size suggests — which brings us to why a thin film costs anything at all.

Which UK arrays actually collect dirt — and which don't

Pitch is the biggest factor, but it is not the only one. In practice the arrays that build up noticeable soiling tend to combine a shallow angle with a local source of muck.

More likely to need attention: flat or near-flat garage and extension roofs; low-pitch bungalow roofs; ground-mounted arrays set at a shallow tilt; anything under or near overhanging trees, which adds leaf litter, sap and a great deal of bird traffic; homes close to busy roads, farmland or building sites; and properties where a chimney, flue or extraction outlet sits upwind of the array.

Less likely to need attention: a steeply pitched main roof in an open suburban or rural setting with no overhanging vegetation. These arrays genuinely do most of their own housekeeping, and in the episode's data overlay it is the steep line that holds steady week after week while the flat line drifts. Moss, lichen and gutter debris are a related but separate issue. They are less about the glass and more about what accumulates around and beneath the array — at the bottom edge, in the gap between roof and module, and in the gutter that the panels now discharge into. That is a maintenance point for the roof as much as the solar system.

A practical checklist: does your array actually need a wash?

Before paying anyone to clean anything, work through this. It takes ten minutes and costs nothing.

1. Identify your pitch honestly. Steep main roof, moderate pitch, low pitch, or effectively flat? If it is flat or near-flat, assume soiling is a live issue rather than a theoretical one.

2. Look at the glass from ground level, ideally with binoculars or a zoomed phone photo taken after a dry spell. You are looking for tide marks, a uniform dull haze, pollen bloom, bird mess and a dark band along the bottom edge of the modules. 3. Compare like with like in your monitoring. Pick two clear, bright days in the same month — one from earlier in the year, one from now — and compare the peak and the shape of the curve. Soiling shows up as a gentle downward drift across weeks, not as a sudden drop. 4. Compare arrays if you have more than one. If your steep roof and your garage roof are on separate strings or inverters, you have a built-in control experiment. Divergence between them over a dry summer is the clearest possible signal. 5. Rule out other causes first. A sudden drop is far more likely to be a shading change, an inverter fault, a tripped isolator or new tree growth than dirt. Soiling is gradual by nature. 6. Time it well. Late spring, after pollen season, and late summer, after a long dry run, are the two moments a wash usually earns its keep in the UK. Cleaning in the depths of December buys you very little generation. 7. Check whether one wash restores the line. In the episode, the flat-roof trace steps straight back up after a single clean. If yours does not, dirt was not your problem — investigate further.

Follow-up questions homeowners ask

Does snow need clearing? Generally no. Snow slides off pitched glass readily as soon as the surface warms, and clearing it involves exactly the roof access you should be avoiding. On a flat array it will sit longer, but it will also melt without help.

Will self-cleaning coatings solve it? Coatings can help water sheet more freely, which is useful — but they cannot make gravity steeper. On a near-flat array, water still has to go somewhere, and if it can only evaporate, the solids still stay.

Can I stop the problem at design stage? Largely, yes, and this is the more valuable conversation. On a flat roof, tilt frames give water somewhere to run instead of somewhere to sit. Sensible module spacing, keeping the array clear of overhanging branches and flue outlets, and thinking about where run-off actually discharges all reduce future maintenance. It is far cheaper to design out soiling than to keep washing it off. Does a dirty panel damage anything? A general film does not damage the module — it just reduces output until removed. Persistent organic matter, moss growth at the frame edge and standing debris are worth dealing with, because they hold moisture against the frame and the roof. Should I clean before checking my monitoring? No. Check first. Data beats guesswork, and knowing what your system looked like before the wash is the only way to know whether it was worth doing.

Video transcript

Mo: So solar panels never need cleaning? RoboMo: Rain rinses steep roofs fairly well. Near-flat arrays hold grime.

Mo: Why doesn't the flat one rinse itself too? RoboMo: Sheeting water carries dust away. Standing water evaporates and leaves it behind.

Mo: So is that actually costing me output? RoboMo: A thin film shades every cell. Steepest roofs stay near clean. Flattest ones repay a wash. Mo: So solar panels never need cleaning? RoboMo: Rain rinses steep roofs fairly well. Near-flat arrays hold grime. / Mo: Why doesn't the flat one rinse itself too? RoboMo: Sheeting water carries dust away. Standing water evaporates and leaves it behind. / Mo: So is that actually costing me output? RoboMo: A thin film shades every cell. Steepest roofs stay near clean. Flattest ones repay a wash.

Wrapping up

"Do solar panels need cleaning?" is really two questions wearing one coat. For a steep, unshaded British roof, rain is a genuinely effective maintenance system and you can largely leave it to get on with it. For a near-flat array — garage, extension, bungalow, shallow ground mount — rain arrives, sits, evaporates and leaves its cargo behind, and a thin film that shades every cell will slowly trim your generation until someone rinses it off. The good news is that this is one of the few solar losses that is fully reversible, cheap to fix and easy to verify: clean it once, watch the line step back up, and you know exactly what it was worth. The better news is that it can be designed out at the start, by giving water somewhere to run rather than somewhere to pool.