24 July 2026

Osmosis water or tap water: why pros use demineralised water

Tap water dries with limescale stains, and that haze is exactly what you wanted gone. How to do it right.

Watch solar panels being cleaned professionally and you will always see the same recipe: osmosis water and a soft brush, no detergent. Osmosis water is water stripped of virtually all dissolved minerals through reverse osmosis: demineralised water. Because nothing is left in the water, nothing is left on the glass after drying either: no limescale stains, no streaks, no residue.

Ordinary tap water does the opposite. Mains water contains dissolved limescale and minerals, and those do not vanish when the water evaporates: they dry into a white haze on the glass. You have brushed away the dust and traded it for limescale: a new light-blocking layer, exactly where you wanted clean glass. That is why hosing down the panels with the garden hose is not a cleaning visit, but at best a relocation of the problem.

The professional solves this with an osmosis-water system in the van and a telescopic brush or soft rotating brushes on the roof. The demineralised water dissolves the dirt, the brush carries it away and the remaining water dries streak-free: without any agent that attacks the panel coating. That last point is no detail: aggressive detergents and abrasive brushes can damage the coating and do more harm than the dirt itself.

But why no detergent at all? Because the problem on a solar panel is almost always loose: dust, pollen, droppings and grime that come off readily with pure water and a soft brush. A detergent adds nothing to that, but does leave something behind: a thin residue on the glass that new dust sticks to more easily, and ingredients that can attack the coating or the rubber seals around the panel. And consider: everything you use on the roof rinses into your garden via the gutter and the downpipe. Pure water is not the frugal option here, it is the better one.

Doing it yourself on a ground-level setup or a safely walkable flat roof? That works fine with the same approach in miniature: demineralised water comes by the jerrycan from the DIY store, and a soft brush or squeegee does the rest. What you never use: a pressure washer, aggressive detergents or a dry, abrasive brush. And pick a cool moment of the day: cold water on sun-heated glass means stress in the panel. If your system sits on a pitched roof or at height, the DIY ends there: that is work for someone with fall protection.

The working order is simple: go panel by panel from top to bottom, wet the glass first so the grime can soak loose, then brush with light pressure and rinse with demineralised water. Do not wipe dry: precisely because nothing is dissolved in the water, it dries streak-free by itself, and with a cloth you are more likely to rub dirt into the glass than out of it. Stubborn spots such as dried-on bird droppings you soak rather than scrub: patience is the tool here.

And the home remedies? Glass cleaner, washing-up liquid and vinegar come up in every forum, but the answer is the same each time: don't. Glass cleaner and washing-up liquid leave exactly the residue film you were trying to avoid, and acidic products do not belong on the rubber seals around the panel. Anything that seems to make the glass shinier than pure water does so by leaving something behind: and on a solar panel, leaving something behind is precisely the problem.

Price-wise the choice is simple: a professional cleaning visit costs € 4 to € 8 per panel, € 75 to € 175 for an average home, including the osmosis water, the equipment and a trained eye for damage or loose cables. If you want it handled structurally, a cleaning subscription is cheaper per visit. Always compare quotes against the price index before you agree.