Solar Basics
Cleaning panels repays in some climates and almost none of them
Rain does the job free above a certain tilt, so paid cleaning only makes sense where soiling is chronic rather than seasonal, and the wrong method costs more than the dirt.
By Kabir Anand4 min read
What dirt actually does to a panel
Soiling reduces output by the simplest mechanism available: it stops light reaching the cells. A film of dust scatters and absorbs some fraction of the incoming light, and the panel produces in proportion to what gets through. If a layer costs you five per cent of the light, it costs you roughly five per cent of the output, and there’s no clever electronics that recovers it.
Uniform soiling is the benign case, because it affects every cell equally and the loss is genuinely proportional. The unpleasant case is patchy soiling, which behaves like shading. A band of moss along the bottom edge, a splash of lichen, or a heavy bird deposit sitting over two or three cells will drag a whole bypass group down, and the loss then greatly exceeds the area covered.
That distinction matters more than any figure. A slightly hazy array is losing a few per cent. An array with a solid line of grime across its bottom row is losing far more than the grime would suggest, because the lowest row of cells is usually a diode group of its own.
Rain is the default cleaner and it works
On a tilted panel, rain runs off and takes loose deposits with it. That process is more effective than most people credit, and it is the reason cleaning contracts are a poor purchase across much of the temperate world. Anywhere with regular rainfall distributed through the year, an array at a normal roof pitch will hover close to its clean output most of the time without anyone touching it.
Tilt is the variable that decides whether self-cleaning works. Somewhere below about ten to fifteen degrees, water stops running off decisively and starts sitting, and a puddle that evaporates leaves everything it was carrying behind. This is one of the practical arguments against very shallow mounting on a flat roof, and it is a stronger argument than the small geometric loss.
One place resists the rain even so. Water running down the glass collects at the lower frame, which acts as a small dam, and deposits build in that strip instead of washing away. Over years a dark line forms along the bottom edge of each module.
Where soiling is a genuine and continuing loss
Arid and semi-arid regions are the clear case. Dust settles daily, rain arrives rarely, and the loss accumulates in a way that never resets on its own. In those climates cleaning is a real maintenance task with a real return, and the interval is measured in weeks or months rather than years. This is a strong latitude and climate dependence, and advice written for a rainy country transfers badly.
Then there are local sources that outweigh the regional climate. Panels beneath a tree that drops sap or blossom. A roof under a flight path used by roosting birds, or one below an overhead cable that birds perch on. Farmland during harvest. Coastal exposure, where salt haze leaves a film that rain removes only partially. A nearby unsurfaced road or a construction site next door.
And there is biology. In damp shaded conditions, moss and lichen colonise the frame edge and creep inward, and unlike dust they don’t wash off. That is the one common temperate case where intervention is genuinely required, and it needs doing before the growth is established rather than after.
The arithmetic of paying somebody
The calculation isn’t complicated, and it is the reason the answer is usually no. Work out what fraction of annual output the dirt is plausibly costing, convert that fraction into units of electricity, value those units at whatever your local import and export prices make them worth, and compare the result against what a visit costs where you live. Both sides of that comparison are local, so no publication can do it for you.
What defeats most temperate cases is that the loss is small and temporary, and the value of the electricity recovered has to cover a trip, access equipment and labour. In a rainy climate a cleaning contract is a way of spending money on your roof rather than earning it.
The picture inverts where soiling is chronic. If dust is taking a substantial and permanent bite from every day of the year, the same arithmetic swings decisively the other way, and the interval becomes an optimisation rather than a question of whether to bother at all.
If you do clean, the method matters more than the frequency
Glass on a panel is toughened and durable, but the surface usually carries an anti-reflective coating, and that coating is the part at risk. Abrasive pads, scouring powders and anything harsher than a soft brush will scratch it, and scratches both scatter light and give dirt somewhere to hold. Detergents leave residues that attract more dirt than they removed.
Two things do real damage. Pressure washing can drive water past the edge seal or the junction box gasket, and moisture inside a laminate is the beginning of delamination and corrosion. And spraying cold water onto glass that has been baking in the sun risks thermal shock, so cleaning belongs to early morning or evening, in cool conditions, which is also when the array is producing least.
The other hazard is not to the panel. Most domestic roofs are not places to stand, and nobody should ever walk on modules, since the microcracks that produces are invisible on the day and expensive years later. If the array can’t be reached from the ground with a water-fed pole, the sensible options are a contractor with proper access equipment or leaving it to the rain. In most climates, leaving it to the rain is the correct answer.
Common questions
How much output does dirt typically cost?
It depends so completely on climate and local sources that a single figure would mislead. In a wet temperate location with adequate tilt the ongoing loss is usually small, and in a dusty arid one without rainfall it can become the largest single loss in the system. The way to find out for your own roof is to watch output before and after a heavy rain shower.
Do coatings that repel dirt work?
Hydrophobic and self-cleaning treatments do measurably change how water and dust behave on glass, and the effect wears off over time in outdoor conditions. Whether the reapplication cost beats the recovered energy is exactly the same local calculation as cleaning itself, and in a rainy climate the base case is already good enough that there is little to gain.
Should snow be cleared off panels?
Usually not by hand. Snow slides off tilted glass on its own once the surface warms, panels are slippery by design, and the risk of damaging modules or falling off a roof outweighs a few days of lost winter output that was never going to be large. Clearing makes more sense on an off-grid system where those days genuinely matter.
Consumer editor, Power Your Roof
Kabir covers solar basics, batteries, bills & tariffs and the questions readers actually send in and is happiest when a piece answers the question completely.
