The Truth About Deionized Water (aka Spotless Water) on Solar Panels

If you've researched professional solar panel cleaning, you may have heard that your panels must be cleaned with deionized (DI), demineralized, or “spotless” water. Deionized water certainly has advantages, especially aesthetically at close distance. But what do solar-panel manufacturers themselves actually say about this?

What Do Solar Panel Manufacturers Actually Say?

Rather than relying on claims made by solar-cleaning companies—including ours—the best place to start is with the companies that manufacture the panels.

LONGi: Municipal Water Can Be Used

LONGi, one of the world's largest solar-panel manufacturers, gives particularly clear guidance in its photovoltaic module maintenance manual:

“Municipal domestic water can be used for the cleaning of PV modules.”

Importantly, LONGi does not say that ordinary municipal water must first be deionized before it can safely clean a solar panel. That distinction matters. Canadian Solar provides similar guidance. Its November 2025 installation manual says that if a module becomes soiled, it can be washed with water and a non-abrasive cleaning implement.

Qcells: Does Not Require DI Water

Qcells takes an even broader approach. Its installation manual states that each customer is generally free to choose the cleaning method, and earlier Qcells documentation specifically instructed owners to rinse dirt from modules with lukewarm water, while avoiding abrasive detergents and inappropriate cleaning equipment. Once again, no requirement for zero-TDS water.

What Actually Matters When Cleaning Solar Panels?

Water quality is only one part of safe solar-panel cleaning.

Manufacturers repeatedly emphasize other factors that are more important:

Avoid abrasive cleaning

Canadian Solar warns that dry dirt should not be scraped or rubbed from the glass because doing so can create micro-scratches. It recommends soft, non-abrasive cleaning implements.

JinkoSolar similarly says never to use abrasive materials when cleaning modules. Interstellar uses the best and softest bristles on the market while cleaning under constant water flow.

Avoid harsh chemicals

Manufacturers generally discourage unapproved chemicals that could damage glass coatings, seals, polymer components, frames, or other module materials. Canadian Solar specifically warns against corrosive cleaning solutions and unauthorized chemicals. Interstellar never uses any chemicals. Period.

Avoid thermal shock

Solar panels can become extremely hot under direct sunlight. Introducing significantly cooler water to a very hot module may subject the glass and other components to unnecessary thermal stress. Interstellar uses specialized tubing that heats up the water as it travels from the water source to the cleaner.

It's really about dirt removal

The National Renewable Energy Laboratory (NREL), a federal research facility under the U.S. Department of Energy, has extensively studied photovoltaic “soiling”—the accumulation of dust, pollen, soot, and other substances on PV modules—and notes that these contaminants can reduce solar-energy production.

NREL research has even found that rainfall may not fully remove certain contaminants such as pollen. At utility-scale sites studied in North Carolina, wet-brush cleaning produced performance improvements after accumulated pollen and biological soiling had resisted rainfall alone.

Can you spot the spotless water?

In our own field experience, the visual difference between panels rinsed with deionized water and panels properly cleaned and rinsed without a DI system has often been difficult or impossible to see from normal viewing distances.

We know this because we previously used a DI system on many homes and were able to compare the finished results. After the panels dried, customers looking at them from ground level could not identify which panels had been rinsed with DI water and which had not.

The Bottom Line

Homeowners should not be led to believe that a company must use expensive “spotless water” equipment for a solar-panel cleaning to be safe or effective. The evidence from major solar manufacturers paints a much more practical picture:

Use suitable water. Use non-abrasive equipment. Avoid harsh chemicals. Clean panels at an appropriate temperature. And most importantly, thoroughly remove the dirt and contamination blocking sunlight.

Those factors matter far more than a marketing claim that the rinse water must contain exactly zero dissolved minerals. At Interstellar Solar Cleaners, our goal is straightforward: clean solar panels safely, effectively, and with sound practices—without selling customers something simply because it sounds more sophisticated.

Sources

This article is based on published guidance from:

• National Renewable Energy Laboratory (NREL), U.S. Department of Energy — photovoltaic soiling and cleaning research

• LONGi Solar — PV Module Maintenance Manual

• Canadian Solar / CSI Solar — Installation Manual and Module Cleaning Guidelines

• JinkoSolar — Global Installation Manual

• Qcells — Solar Module Installation and Operation Manual

Always consult the cleaning and maintenance instructions supplied by the manufacturer of your specific solar modules, because requirements can vary by model and warranty.