Solar panels sit outside year in, year out, and after a while you can tell: a rim of dirt along the bottom of the glass, a few white bird droppings, green growth along the frame. The question is whether that matters. The honest answer: often not, sometimes yes. This article explains when cleaning makes sense and when the rain does the job, how to tell from the yield that something is going on, how panels are cleaned safely without damaging the coating, why as a private individual you are better off not going up on the roof yourself, and who can do the work for you. You can read more about installing panels, choosing the inverter and the guarantees in Having solar panels installed; this article is about what comes after that.
Do solar panels actually need cleaning?
On most Dutch roofs, not at all or hardly ever. Milieu Centraal (the Dutch independent information centre on energy and the environment) writes that the rain washes solar panels clean by itself if they are tilted at more than 20 degrees, so that cleaning is then in principle unnecessary. An ordinary tiled roof is usually steeper, so on a pitched roof a rain shower is generally enough.
That is different for panels that lie (almost) flat. There, water stays on the glass, and whatever is in it dries and accumulates, especially against the bottom edge of the frame. Milieu Centraal recommends cleaning panels that lie (almost) flat, or having them cleaned, at least once a year. On flat roofs, panels are often set at a low angle, for example in an east-west arrangement. According to the Consumentenbond (Dutch Consumers' Association), rain washes panels at an angle of less than 20 degrees clean less effectively, so occasional cleaning can then be useful.
Even on a pitched roof, cleaning may be needed if there is stubborn dirt. Milieu Centraal cites bird droppings and algae growth as examples of stains that a rain shower does not wash away.
What soils panels and costs yield?
A solar panel generates electricity from the light that passes through the glass onto the solar cells. Anything that blocks that light costs yield. Exactly how much depends heavily on the type of dirt, the amount and the location. The main culprits are:
- Bird droppings. A single spot looks small, but it completely covers part of a cell. Because the cells in a panel are connected to each other, such a spot can reduce the panel's output more than its surface area would suggest.
- Moss and algae. Especially in places with lots of shade and moisture, such as under trees, and on panels with little tilt, green growth develops along the frame and creeps up the glass from the bottom edge.
- Pollen and leaves. In spring and summer, pollen can leave a yellow film; in autumn, leaves stay on flat panels and between the panels and the gutter.
- Dust from farming, industry and traffic. Next to fields, livestock sheds, a busy road, the railway or an industrial estate, a grey or greasy layer that the rain does not fully wash away builds up faster.
- Salt near the coast. Close to the sea, salt can settle on the glass, similar to the deposits on windows.
Many percentages for yield loss due to dirt circulate on the internet. They vary widely and depend on the situation, so do not rely on them blindly. What is certain: visible dirt on a steeply tilted panel that the rain can reach easily usually costs little, whereas a thick layer on a flat panel, a panel covered in bird droppings or a panel half buried under leaves produces noticeably less.
The inverter app as a signal
Whether your panels need cleaning is best judged not from the street but from the figures. Many inverters have an app or web portal in which you can track daily, monthly and annual yield; Milieu Centraal points out that there are apps and websites that show your panels' performance, and recommends also regularly comparing the meter readings with the expected yield.
Above all, compare the same month in different years, because yield varies greatly with the season and the weather. Also check whether all panels or strings perform roughly the same, if your system shows this per panel. Milieu Centraal advises having the panels and the inverter checked if the electricity yield drops by more than 10 per cent. Note: dirt is only one of the possible causes. Milieu Centraal names a deteriorating inverter and dirty panels as common causes of lower yield, and gives an average inverter lifespan of 12 years. New shade from a tree or a neighbour's extension, a loose connector or an inverter that throttles back when grid voltage is high can also reduce yield. So if there is an unexplained drop, have not only a cleaner but also the installer take a look.
How panels are cleaned safely
Good cleaning is above all gentle. Professionals usually work with demineralised water or osmosis water, water from which virtually all limescale and minerals have been removed, and a soft brush on a telescopic pole or a sponge. Because there is no limescale in the water, it dries without spots and the glass does not need to be squeegeed. It is the same technique many window cleaners use for windows, as described in Having your windows cleaned.
What you do not want is just as important:
- No pressure washer. According to the Consumentenbond, a pressure washer can damage the dirt-repellent coating. The powerful jet can also force water past the frame seals and into the connectors and cabling, where it does not belong.
- No aggressive cleaning products. Many panels have an anti-reflective or dirt-repellent coating on the glass. Cleaning products, solvents and abrasive products can attack that coating, which actually reduces yield.
- No scrubbing or scraping. A hard brush, scouring pad or scraper scratches the glass and coating. Baked-on dirt, such as old bird droppings, is soaked first and then removed carefully.
- No standing or leaning on the panels. According to the Consumentenbond, leaning on or walking over panels can cause invisible damage, such as microcracks in the cells. The fire service's guidance card also states that you should never put pressure on an intact solar panel.
- No cold water on a hot panel. On a sunny day, panels get hot. Manufacturers often advise in their cleaning instructions to clean when the panels are cool, for example early in the morning or in cloudy weather, to prevent stress in the glass from a large temperature difference. The water also dries less quickly then.
Before cleaning, check the manufacturer's warranty terms and manual. Manufacturers often state which methods and products are permitted, and damage caused by an unsuitable cleaning method may fall outside the product warranty. Ask the cleaner whether they work according to those instructions.
Why as a private individual you are better off not going up on the roof yourself
Cleaning panels seems a simple job, but the work takes place on a roof: high up, often sloping, sometimes slippery with moss or wet from the water you are working with. According to the RIVM (the Dutch National Institute for Public Health and the Environment), consumer products play a role in some 60,000 accidents a year requiring emergency care, and about five per cent of the victims fall from a portable stepladder or ladder. And a fall from a roof is usually more serious than a fall from a stepladder.
On a flat roof without railings or edge protection, the edge is quickly closer than you think, especially if you step backwards with your eyes on the panels. On a pitched roof you have nothing to hold on to without fall protection, and you must not use the panels themselves as a foothold. A ladder against the gutter is not a stable base either. If the panels are low down, for example on a shed, a carport or an extension, you may be able to reach them from the ground or a stable stepladder with a telescopic pole. Anything you can only reach by going up on the roof is best left to a professional. Which jobs you can safely do yourself is explained in What you can do yourself in a renovation and what you had better not.
What applies to the professional on the roof
For anyone working on a roof professionally, safety is regulated by law. The Arbobesluit (Dutch Working Conditions Decree) requires employers, and for these rules also self-employed people without staff, to take measures against the risk of falling (Article 3.16 and Article 9.5 of the Arbobesluit). Where the potential fall height is 2.5 metres or more, under the Arbobesluit there is in any case a risk of falling and measures are mandatory; measures must also be taken at a lower fall height if there is a risk of falling. Collective measures, such as railings, edge protection, scaffolding or an aerial work platform, take precedence over individual measures such as a harness attached to an anchor point.
Under the rules, a ladder is primarily a means of getting somewhere, not a workplace. A ladder may only be used as a workplace at height if safer work equipment is not justified, because of the low risk and either the short duration of the work or features of the location that cannot be changed. A serious cleaner therefore comes with an aerial work platform, works with a telescopic pole from the ground or a safe standing position, or sets up edge protection or a fall-arrest system on the roof. Ask in advance how they will reach your panels. Anyone who walks across a pitched roof without any provision is not a tradesperson to hire, however attractive the offer sounds.
Electrical safety: panels are live
A solar panel has no on/off switch. As long as light falls on it, it produces voltage, even if the inverter or the circuit in the meter cupboard has been switched off. The fire service points out that voltage always remains on the solar panels when light falls on them. An undamaged system is safe to touch, but damaged, melted, falling or loose panels may be live. Wet conditions increase the risk.
For you, that means: do not touch a panel with broken glass, a scorched spot or a cable hanging loose across the roof, and do not spray water on it. Report it to the installer or an electrician, who can safely switch off and repair the system. A cleaner who comes across such damage should stop working on that panel and report it to you.
More than cleaning: inspection during the visit
A cleaning visit is a good opportunity to have the panels looked at up close, because you cannot get to them yourself. Ask the cleaner to take photos and to look out for:
- The panels themselves: cracks or star-shaped chips in the glass, discolouration of the cells, edges coming loose or moisture behind the glass.
- The mounting: loose clamps, shifted panels, shifted or broken roof tiles around the hooks and, on a flat roof, shifted ballast or damaged roofing under the frame.
- The cabling: cables hanging loose across the roof or gutter, gnawed insulation or connectors lying in a puddle of water.
- Birds under the panels: the space between panels and roof tiles is a popular nesting place for pigeons and other birds. Nesting material, droppings and birds pecking at the cables cause soiling and damage. Pigeon proofing made of mesh or a comb around the panel array keeps them out. Nests of wild birds are protected by law (Article 11.37 of the Besluit activiteiten leefomgeving, the Dutch Environmental Activities Decree). For most species that protection applies as long as the nest is in use, but for species including the house sparrow and the common swift, permanent nesting sites are protected all year round, and those are precisely the species that like to nest under roof tiles. So have proofing fitted when no birds are breeding, and if you are unsure about the species or there is an occupied nest, consult an ecologist first.
- Dirt in the gutter: leaves and dirt washed off the panels end up in the gutter. How to keep the gutter clean and intact is explained in the maintenance calendar for your home.
A cleaner is not an installer. If they see technical defects, the repair belongs with the installation company or an electrician. The same applies to work that requires panels to be removed, for example to remove a nest or fit proofing underneath.
Insurance and warranty
With many insurers, solar panels on the roof fall under the opstalverzekering (buildings insurance), because they are fixed to the home, but the terms differ per policy. Report the panels to your insurer if you have not already done so, and check whether damage from storms, hail or fire is covered and whether there are requirements for maintenance and installation. Damage due to overdue maintenance is excluded under many policies, and cover may also be limited for damage caused by unprofessional work; that differs per policy. If you have the panels cleaned, ask whether the cleaner has business liability insurance for damage they cause to your panels, roof or gutter. You can read more about insurance for work on your home in Renovating and insurance.
The manufacturer's and installer's warranty may also depend on how the panels are treated. So keep receipts and reports of cleaning and inspection visits, including the method used, so that you can show in a warranty claim that maintenance was carried out according to the instructions.
Who does it: window cleaner, specialist or installer?
For the cleaning itself, you can turn to two kinds of firms:
- The window cleaner. Many window cleaners already work with osmosis water and telescopic poles and offer solar panel cleaning as an extra service, often combined with the windows or the gutter. For panels on a pitched roof that can be reached from the ground or an aerial work platform, that is a logical choice. Via VerbouwPro you can find a window cleaner in your region.
- A specialist cleaner. For large panel arrays, flat roofs with many panels or roofs where work has to be done with fall protection, there are firms that specialise in solar panels.
For the technical side, the installation company or an electrician is the right party: a drop in yield that is not caused by dirt, an inverter showing error messages, loose cables, damaged panels or a panel that needs to be removed. Some installers offer a maintenance visit combining inspection, measurement and, where needed, cleaning. If the roofing under or around the panels is damaged, a roofer should be involved.
How often, and what belongs in the quote?
There is no fixed rhythm for every roof. As a rule of thumb: panels on a pitched roof in a clean environment often need cleaning never or only rarely, (almost) flat panels at least once a year, and panels under trees, near livestock sheds, fields, industry or in an area with many birds as often as the yield and their appearance call for. Let the inverter app set the pace rather than the calendar. A clean at the end of spring, after the pollen season, or in autumn after the leaves have fallen, is the obvious choice.
Preferably ask for a quote based on the number of panels, the type of roof and accessibility, and check that it includes:
- the number of panels and their position (pitched or flat roof, height, accessibility);
- the method: osmosis water, soft brush, no pressure washing and no cleaning products;
- how the firm works safely: aerial work platform, telescopic pole, edge protection or a harness;
- whether a visual inspection with photos and a short report is included;
- what happens if damage or nests are found, and whether pigeon proofing can be fitted;
- the business liability insurance and registration with the KvK (Chamber of Commerce).
In short: on a pitched roof the rain does most of the work, on a flat roof it does not. Track your yield in the app, if there is a clear drop first have someone find out whether dirt or the technology is to blame, and leave the work on the roof to a professional who cleans gently and works safely. On VerbouwPro you post your job free of charge and window cleaners and electricians from your region respond.