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Near-flat solar panels may stay dirtier because rain and wind do not clear them as effectively as they clear steeper modules. That makes soiling one possible reason for a production dip—not proof of its cause. Tap water is not automatically off-limits, but its minerals can leave spots if they dry on the glass. Check the exact panel manual before cleaning, and do not risk a fall to reach the array.
Why near-flat panels may collect more dirt
Dust, pollen, bird droppings, and other deposits can remain on a shallow panel surface when rain and wind do not remove them. PREFA notes that natural cleaning may be insufficient in some cases, including low-pitch installations. A module manual dated 6 November 2019 says flat-mounted modules need more frequent cleaning than modules tilted at 10° or more because they self-clean less effectively. That comparison is specific to that manual, not a universal cutoff for every panel.
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For an array in a dry, windy part of southern Spain, that makes accumulated dirt plausible, but neither the location nor the tilt alone establishes what caused a particular system’s lower output. A solar-forum post describes a carport or pergola array at about 3° tilt in southern Spain, but that is one owner’s account, not evidence that all such systems have the same problem: the discussion.
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- Check the inverter or monitoring history for a sustained change rather than judging from a single low-production day.
- From a safe place, look for visible dust, pollen, bird droppings, or dried deposits. Do not climb onto or stand on the panels to inspect them.
- Identify the panel manufacturer and model, then read its current cleaning and warranty instructions before choosing water, tools, detergents, or pressure.
- If cleaning is permitted and safe, compare generation afterward with periods of reasonably similar sunlight and operating conditions. If output remains low, ask the installer to investigate instead of repeatedly washing the panels.
Visible dirt can affect system performance, according to REC’s cleaning guide, but an output dip alone cannot identify soiling as the cause. The available information does not establish the cause of any particular installation’s drop or a specific amount of output that cleaning would restore.
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Can tap water leave limescale spots?
It can. Dissolved minerals—especially calcium and magnesium, which contribute to water hardness—can leave spots when rinse water dries on glass. The displayed text of BS EN IEC 62446-2:2020 describes those minerals as contributors to spotting. PREFA generally recommends treated water. REC says deionized water is ideal, while rainwater or tap water may be used as a secondary option when deionized water is unavailable.
Those instructions are not interchangeable rules for every panel. Whether tap water is appropriate depends on the local water, whether residue is allowed to dry, the panel design, and the manufacturer’s directions. Avoid a universal household hardness cutoff: the cited standard excerpt does not establish one.
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How to clean only if the manual and access allow it
- Choose suitable conditions. REC advises cleaning when panels are cool and using water at ambient temperature. One dated module manual specifies irradiance below 200 W/m² and warns against a large temperature difference between the cleaning liquid and module. Follow the instructions for your own model rather than treating that threshold as universal.
- Use only permitted water and tools. REC lists a soft sponge, microfiber cloth, or non-abrasive brush for stubborn marks. PREFA recommends treated water and a soft cloth or sponge. Do not assume a detergent or a particular water source is approved unless the manual says so.
- Do not leave mineral-rich rinse water to dry when practical. This can reduce the chance of mineral spotting; it does not guarantee spotless glass.
- Do not walk on panels or use high-pressure cleaning without explicit approval. PREFA and REC prohibit high-pressure cleaning for their products. The dated manual has a separate pressure limit, illustrating why the exact product instructions matter.
REC describes dirt as not harmful by itself but warns that allowing it to build up can affect system performance. That is not permission to use a method that could damage a particular module.
When to hire a qualified cleaner
Choose professional help if the array is on a roof or otherwise hard to reach, if safe access is uncertain, or if the product instructions require qualified personnel. PREFA says its PREFALZ solar panels should only be cleaned by qualified personnel. REC likewise advises assessing access and seeking professional advice when uncertain.
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There is no established universal break-even price for hiring a cleaner; costs depend on the installation and local service. The practical choice is whether safe access, the maker’s requirements, and the visible condition of the panels justify doing it yourself. If output stays low after permitted cleaning, an installer’s diagnosis is more useful than another cleaning attempt.
What the cited guidance does—and does not—establish
REC and PREFA provide manufacturer-specific cleaning guidance; the PS-M-0694 manual is dated 6 November 2019 and applies to its own product; and the displayed standard excerpt discusses soiling and cleaning considerations. They support checking the product instructions, using suitable conditions and soft tools, and treating mineral residue as a real possibility. They do not establish a Spain-wide soiling-loss percentage, diagnose an individual system, or provide a homeowner-specific test protocol.
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