Why I Clean Solar Panels With Deionized Water

Half of a panel cleaning is the water, and nobody talks about that part. The brush gets the attention. But what you rinse with decides what the glass looks like after it dries — and the panel manufacturers have opinions about it, in writing.

What deionized water is

Deionized (DI) water has had its dissolved minerals stripped out. Tap water hasn't. When tap water dries on hot glass, the water leaves and the minerals stay — as spots, streaks, and a faint crust that builds up visit after visit. DI water dries clean because there is nothing in it to leave behind.

That is also why DI water is an effective cleaner on its own for most soiling. With no minerals of its own, it picks up and carries off the dirt, pollen, and residue on the glass instead of trading one film for another.

What the manufacturers specify

This is not my preference dressed up as science. It is what the manuals say:

• REC is the strictest: "use only deionized water" free of grit and contaminants, applied with a sponge, microfiber cloth, or soft brush — and high-pressure hoses are not permitted at all, because they can damage panels, laminate, or cells. (From REC's installation manual, quoted in the International Solar Cleaning Association's compilation.)
• Canadian Solar requires water of low mineral content with a pH between 6 and 8 for wet cleaning.
• Silfab calls deionized or distilled water ideal for cleaning its panels.

Read across the major brands and the pattern holds: low-mineral water, soft contact, low pressure. My rig is built to exactly that spec — a mobile deionized-water system and a soft rotary brush, run at low pressure, with the method matched to whatever the manufacturer of your panels instructs. Where a manual restricts a tool, the manual wins; brand-specific limits are covered in the warranty post.

One honest note on soap: for heavy soiling — baked-on droppings, sap, a season of farm dust — I use a biodegradable panel soap to break the bond, then rinse with DI water. Any cleaner who promises you a process with zero soap ever is either not cleaning heavily soiled arrays or not telling you something. Soap used rarely and rinsed fully is different from detergents baked onto glass by a pressure washer, which is what the manuals prohibit.

The NREL test that settled the hose question

If water is half the job, is a garden hose enough? NREL tested something close to that question on bio-soiled panels from a rainy-climate system in the southeastern US — panels with a dark fungal-type film that did not look visibly dirty. Against a clean control, the dirty modules measured 13.4% and 14.6% maximum-power losses.

Then they cleaned, two ways:

• Hose water alone on the worst module: loss went from 14.6% to 12.9%. Barely moved.
• Soft-bristle brush plus hose water: loss down to 8.1%. Much better.
• Neither method fully restored the panel. Sticky, cemented soiling does not come all the way back with light treatment — which is an argument for cleaning before soiling cements, not for skipping it.

That is the whole method in one experiment. Water alone rinses. Contact plus the right water cleans. It is also why rain doesn't clean solar panels, no matter how hard it falls.

Why not just pressure wash it?

Because pressure is the other half of the manuals' warnings. REC prohibits high-pressure hoses outright, Canadian Solar caps water pressure in its cleaning spec, and Q CELLS bans high-pressure water equipment entirely. The reasoning is physical: a panel is glass over cells, seals, and a laminate, and a pressure jet drives water where it was never meant to go while stripping at the surface. Everything soiling does slowly, pressure can do in one afternoon — and damage from it falls under the same "inappropriate cleaning" warranty exclusions covered in the warranty post. Low pressure and soft contact take a few minutes longer. They are also the whole point.

What this means for your array

If your panels were installed in southern Wisconsin, they are being rinsed by weather with whatever the sky carries and dried by sun that bakes the residue in. A DI-water cleaning resets the glass to what the manufacturer intended — no mineral crust, no detergent film, no pressure damage. It is also, per REC's manual, literally the only water allowed on their panels.

Want to know what a cleaning costs for your panel count? The rates are published — no form, no phone tag — on the cost page. Or text me at (608) 571-4173 and I'll quote your roof from satellite imagery, free.