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Near-flat solar panels can hold onto dust and other grime, so soiling is one possible reason for lower output—but a production dip alone does not prove that dirty glass is to blame. Tap water is not automatically unsuitable: mineral-rich water can leave spots, while some manufacturers allow it as a fallback. Check your panel’s own instructions before cleaning, and do not risk a fall to reach the array.
Why near-flat panels may collect more dirt
Rain, wind and snow do not necessarily clear a low-pitched module as effectively as a steeper one. PREFA notes that natural cleaning may be insufficient in some cases, including low pitch. A module manual dated 6 November 2019 says flat-mounted modules should be cleaned more often because they do not self-clean as effectively as modules installed at 10° tilt or greater. That comparison comes from that specific manual; it is not a universal cutoff for every panel.
In a south-of-Spain carport or pergola with very little rain, visible dust, pollen, bird droppings or dried deposits may therefore be worth checking. But neither a low tilt nor a dry climate establishes that soiling caused a particular system’s decline.
How to check a production dip before cleaning
- Review the inverter or monitoring history for a sustained change, rather than treating one low reading as a diagnosis.
- From a safe location, look for visible dirt or deposits on the modules. Do not climb onto or stand on panels to inspect them.
- Identify the module brand and model, then read its current cleaning and warranty instructions. Water, tools and pressure rules vary by manufacturer and product.
- If cleaning is permitted and safe, compare generation afterward only with periods of reasonably similar sunlight and operating conditions. If output remains low, ask the installer to investigate rather than repeatedly cleaning the array.
Accumulated visible dirt can affect system performance, according to REC’s cleaning guide, but the reviewed sources do not establish the cause or quantify the recovery for any particular installation.
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Does tap water leave limescale on solar panels?
It can leave mineral spots, depending on the water and whether it dries on the glass. The displayed text of BS EN IEC 62446-2:2020 identifies dissolved calcium and magnesium as contributors to water hardness and spotting. That explains why rinsing with hard water and letting it evaporate can leave a visible residue; it does not establish a universal household hardness threshold for deciding whether tap water is safe.
Manufacturer advice is not identical. PREFA generally recommends treated water and a soft cloth or sponge for its products. REC says deionized water is ideal, but rainwater or tap water may be used as a secondary option if deionized water is unavailable. Follow the instructions for your exact module rather than treating either maker’s guidance as a rule for all panels. When permitted, avoid leaving mineral-rich rinse water to dry on the surface where practical.
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If cleaning is allowed, use the maker’s method
- Choose suitable conditions: REC advises cleaning when panels are cool, using water at ambient temperature, ideally before the modules reach working temperature. A manual dated 6 November 2019 specifies irradiance below 200 W/m² and warns against a large temperature difference between the cleaning liquid and the module. Those figures and conditions belong to that manual; use your own module’s instructions.
- Use gentle tools: REC lists a soft sponge, microfiber cloth or non-abrasive brush for stubborn marks. Avoid abrasive tools or detergents unless the manufacturer explicitly permits them.
- Do not assume pressure washing is acceptable: PREFA and REC prohibit high-pressure cleaning for their respective products. The cited module manual has its own pressure limit, another reason not to transfer one manufacturer’s method to another product.
- Stop if access is unsafe: Never stand or walk on modules. REC advises assessing access and seeking professional advice if uncertain. PREFA says its PREFALZ solar panels should only be cleaned by qualified personnel.
Choose DIY cleaning or a qualified cleaner
| Choice | When it may fit | What to check |
|---|---|---|
| Owner cleaning | The manufacturer permits it, the modules are safely reachable, and the dirt is visible. | Follow the exact manual for water, tools, timing and pressure. Do not climb onto the array or proceed if access is uncertain. |
| Qualified cleaner | Access is difficult, the system uses an integrated product that requires qualified personnel, or you are unsure how to clean it safely. | Ask what method and water treatment the service will use, and confirm that it is compatible with the module maker’s instructions. Local prices and a universal cost break-even point are not established by the cited sources. |
Cleaning can address visible soiling; it cannot establish or fix every cause of reduced generation. If the monitoring history stays low after a permitted, safe cleaning—or the panels do not look dirty—have the installer assess the system.
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Sources and scope
- PREFA cleaning instructions cover its own products, including PREFALZ solar panels.
- Solar-module manual PS-M-0694, Version C is dated 6 November 2019; its thresholds are product-specific.
- REC’s quick cleaning guide gives guidance for REC panels.
- Displayed BS EN IEC 62446-2:2020 text describes soiling and cleaning considerations. Use the licensed standard for formal compliance.
- IDAE’s 2011 grid-connected PV technical specification provides installation context; it is not the source of the product-specific cleaning advice above.
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