Pure water is water with the dissolved minerals taken out. It lifts dirt off the glass and then dries without leaving anything behind.
What is pure water?
On our trucks, it is deionized water: tap water passed through filters and a resin that pull out almost everything dissolved in it. What comes out the other end is close to plain H₂O, with near zero dissolved solids.
It looks exactly like tap water. It tastes flat. On a panel, the difference is what you do not see after it dries.
We call it pure water on this site because that is what it does, not because it is a special product. It is a well-understood way to clean glass, and it is what panel makers ask for.
Why does tap water leave spots?
Because tap water is never just water. It carries dissolved minerals picked up on the way from the reservoir or the well: calcium, magnesium and others, in amounts that vary by utility and by neighborhood.
When a drop of tap water dries on glass, the water leaves and the minerals stay. Each drop becomes a faint white ring. Rinse an array with a hose on a warm morning and you can end up with hundreds of them, a pattern of hard water spots right where the light needs to pass.
Lawn sprinklers do the same to ground mounts, one fine mist at a time, every morning of the summer.
How does the truck make it?
In stages, from the tank to the brush:
- Sediment filter: catches sand, rust and grit before anything else.
- Deionizing resin: swaps the remaining mineral ions for hydrogen and hydroxide ions.
- Meter on the line: reads dissolved solids, so the crew knows when the resin is spent.
The clever part is the resin. Mineral ions in the water stick to the resin beads and trade places with hydrogen and hydroxide ions. Those two then combine into ordinary water. The minerals stay in the cartridge, not on your glass.
When the meter starts to creep up, the resin has used up its capacity and gets replaced. Water that reads high never goes on a panel.
Does dry Colorado air matter?
It makes the spotting problem worse. Front Range air is dry for much of the year, and a mile up the sun is strong. A drop of water on glass here does not linger. It shrinks and dries in minutes, and every mineral it carried is left exactly where it stopped.
In a humid place, a rinse has time to run off before it dries. Here, especially on a breezy morning, tap water can dry in streaks before it reaches the bottom frame.
Pure water turns that into an advantage. Because it carries nothing, fast drying is harmless. The glass is clear the moment it is dry, and on a cool Front Range morning that is usually within the hour.
Why does pure water clean better?
Two reasons. First, it has room to carry what it picks up. Water already full of minerals has less capacity to hold dust and film. Pure water dissolves and suspends more of what is on the glass, then carries it away down the panel.
Second, it dries clean. Because there is nothing in it to leave behind, there is no need to squeegee or towel the glass dry. That matters, because a squeegee drags any missed grit across the surface. We let the water run off and the sun finish the job.
It does not do magic. Pure water will not dissolve tree sap or a dried bird dropping on its own. It just means the rinse, the last thing to touch the glass, leaves nothing new.
What do panel makers say?
They ask for it, or for something close. Panasonic’s EverVolt manual specifies deionized water free of grit, at ambient temperature, with a sponge, microfiber cloth or soft brush.
REC’s cleaning guide says panels must be cleaned with water at ambient temperature, and that the warranty is valid only if the installation manual’s instructions are observed.
So the water is not only about a nicer finish. On some panels it is part of what the warranty expects. Our page on warranties and cleaning sets out what the main manuals ask for side by side.
Why at air temperature?
Because Front Range glass gets hot. On a clear summer morning a roof panel warms quickly once the sun is on it, and by midday it can be far hotter than the air.
Cold water on hot glass makes the surface shrink faster than the layers beneath it. That is thermal shock, and it is the reason both REC and Panasonic say “ambient temperature.” Our water rides in a tank on the truck and sits at the temperature of the morning.
We also pick the hour. Washes start early, when the glass is cool and the dew helps loosen the film, or late in the afternoon once the roof has cooled. We do not wash hot glass in full midday sun.
Is it safe for plants, pets and roofs?
Yes. There is no soap, no detergent and no additive in it. What runs off the roof is water plus whatever was on the panels: dust, pollen, droppings and leaf bits.
We use about 1 to 2 gallons per panel. On a 25-panel roof, that is a modest amount spread across a morning, much of it soaking into the lawn or the gravel along the drip line.
Pure water does not harm shingles, gutters or plants. If anything, the pollen and grime it carries away are the less pleasant part.
Is pure water enough on its own?
For the rinse, yes. For the cleaning, it needs help. The full method on every panel cleaning visit pairs it with the right tool for each kind of soiling:
- Film and dust: soft-bristle brushes on water-fed poles, worked top row down.
- Bird droppings: soaked first, then lifted with microfiber by hand.
- Sap and pitch: soaked and worked gently, never scraped with a blade.
- Old mineral rings: softened over several rinses; very old ones may stay faint.
What we never add is pressure. Our article on why we never pressure wash solar panels explains what a jet can do to seals and coatings.
Can you use pure water yourself?
Yes, in a small way. If you rinse a ground mount or a single-story array yourself, you have a few options:
- Distilled water from the store: fine for spot-cleaning a few panels with a soft cloth.
- A small inline filter: some hose-end units soften or deionize water for small arrays.
- Tap water, then nothing: if you must, rinse early on cool glass and accept some spotting.
What you should not do is towel a roof dry from a ladder, or squeegee hot glass. Our guide to cleaning panels yourself, safely covers the rest, including when a roof is not a do-it-yourself job.
Does rain count as pure water?
Rain starts close to pure, but it picks up dust on the way down and on the glass. A good soaking storm rinses steep panels well. Light showers can do the opposite and paste dust in place. We go into that in does rain clean solar panels?
The short version: rain is a free partial rinse. It does not lift droppings, sap or the line of grime at the bottom frame.
What does pure water look like afterwards?
Clear glass. No rings, no streaks, no haze from soap. In morning light the panels go back to the deep, even blue or black they had when they were installed, and the frame edges come up clean.
Then we check it the only way that counts. The monitoring desk compares the next clear days in your app against the 30-day baseline we pulled before the visit, and you get the comparison along with photos of every row.
Where to go from here
If your panels show white rings or a dull film, start by seeing whether it is costing you much with the output check. When a wash makes sense, you can book a morning and we will bring the water.