Should You Wash Your Solar Panels, and What Can Crack the Glass?
Summary
REC, the panel maker, recommends cleaning when dirt can be seen on the glass and gives no fixed schedule; at a sufficient tilt, ordinary rainfall cleans panels on its own. When washing is needed, REC's guide calls for cool panels in the early morning, water at ambient temperature, and a soft sponge or microfiber cloth. Pressure washers, steam cleaners, and metal sponges void its warranty.
Temperature is the hidden risk
In normal operation, panels run around 20°C above the surrounding air, according to REC. Hit a hot panel with cold water, or a cold panel with hot or boiling water, and the thermal shock can crack the glass. That is why the guide limits cleaning to times when the panels are cool, before they reach working temperature, and to water at ambient temperature.
What REC allows and what it rules out
Allowed:
- a soft sponge, microfiber cloth, or soft nonabrasive brush, which can go on an extension pole
- a standard garden hose at domestic water pressure
- de-ionized water ideally, otherwise rainwater, tap water, or diluted alcohol, free of grit
- a mild biodegradable washing-up liquid, rinsed off right away with plenty of water, or isopropyl alcohol under 10%
Not permitted, and warranty-voiding on REC panels:
- pressure or steam cleaners and high-pressure hoses
- knives, blades, and metallic sponges
Acid or alkali detergents are also ruled out. Air drying is best; otherwise a chamois or a rubber squeegee with a plastic frame, drawn from top to bottom.
Nobody should stand, walk, or lean on a panel, or press on it; the concentrated load can exceed its certified design load. A damaged panel can carry a lethal shock hazard from leakage current, and the risk rises when it is wet. Snow, REC adds, does not have to be cleared for maximum performance.
How much dirt costs
Dirt does not harm the panel itself, REC says, though a buildup can affect performance. For modeling, NLR's PVWatts manual (2014, published when the lab was still NREL) uses a default soiling loss of 2%, one line within a default total system loss of 14%.
Sources
- REC Group — Quick Guide to Cleaning
- NREL (now NLR) — PVWatts Version 5 Manual (NREL/TP-6A20-62641, 2014)
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