Hard water and solar panels: is purified water needed?

Why does water leave marks?

Tap water contains dissolved minerals. When drops dry on warm glass, minerals remain as visible white spots. Cleaning in direct sunlight increases the chance of rapid evaporation.

Do the marks harm the panels?

A few faint marks are mainly cosmetic. Repeated washing with very hard water, especially on hot glass, can allow deposits to build into a haze that admits less light and is harder to remove.

Do not scrape deposits or use acidic or abrasive products without the manufacturer's approval: the glass and its coating could be damaged.

Is tap water suitable?

Often, yes. Check your panel manufacturer's guidance. Work while panels are cool, use enough water to rinse loosened dirt thoroughly and allow gentle air drying. Avoid letting soapy or abrasive products leave a film.

When purified water helps

For very hard water, extensive panel areas or repeated visible residues, osmosis-treated or demineralised water can reduce spotting. It removes much of the dissolved mineral content before rinsing; it does not replace safe brushing where dirt is stuck.

A simple residue-conscious method

1. Consult the panel manufacturer's instructions.
2. Choose a cool time of day, not bright sunshine or frost.
3. Pre-rinse at low pressure, then use a clean soft PBT brush without forcing it.
4. Rinse thoroughly with suitable water and let the modules air dry.

Getting a thorough rinse

Clean cool panels early or late in the day, or under cloud cover, so water drains rather than drying immediately. Rinse generously and work row by row, finishing each section before it dries. Avoid warm, dry wind.

A secure garden tap connection gives an even flow. Both our PBT telescopic brush and motorised double-disc brush have a built-in water supply, allowing you to brush and rinse together. If using a purification system, check its connection to the brush hose before buying.

Key points at a glance

• Minerals in tap water can leave white marks when the water dries.
• Osmosis-treated or demineralised water leaves very little residue.
• For a small home installation, tap water can be sufficient if you work on cool glass and rinse thoroughly.
• Purified water may help for large areas or very hard local water.
• Do not use scrapers, acid or aggressive chemicals.

Unsure about your water or which method suits your panels? Send photos of the panels and water connection and ask us for advice.

What to avoid

Never use abrasive descalers, metal scrapers or a pressure washer on panels. If marks persist, ask your installer or a cleaning specialist before using chemicals.