Updated
Panels Covered In Moss, Dirt Or Lichen
A green or black band has built up along the bottom edge of every panel and nothing rinses it off.
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What's happening?
Rain keeps the middle of a panel reasonably clean, so soiling concentrates in a predictable place: the bottom edge, where the frame forms a lip that water and dirt sit against, and where moss and lichen then take hold. That band matters more than its size suggests, because a strip of shading across the bottom of a panel can affect the whole cell row above it and, on a string system, drag on every panel wired with it. Shallow-pitched arrays suffer more because water sits rather than runs. So do arrays under trees, near farmland, on coastal exposures and beside busy roads. Cleaning is genuinely worthwhile in those conditions and largely pointless on a steep, exposed roof that rain already washes. The real question is not whether to clean but who does it and how: this is work at height over a live DC array, with a coating on the glass that harsh chemicals and abrasives destroy.
What usually causes it?
- A shallow roof pitch, so rain does not run debris off the glass
- Overhanging trees dropping sap, leaves and bird droppings onto the array
- Moss and lichen from the roof itself migrating onto the panel frames and glass
- Agricultural dust, coastal salt or traffic film building a persistent film
- Birds nesting under or above the array, leaving droppings across the glass
- An array that has never been cleaned in a decade of service
Is it dangerous?
The cleaning is the hazard, not the dirt. Roof work is the biggest single cause of serious falls in this kind of maintenance, which is why HSE guidance on work at height matters here, and the array below your feet is live DC in daylight. Two more specific risks: high-pressure jetting can force water past panel seals and is a known cause of failures, and walking on panels cracks cells invisibly under the glass. Never go up a ladder to clean panels yourself.
Can I fix it myself?
Judge whether it is worth doing, and gather evidence for whoever quotes. Photograph the array from the ground with a zoom, look for the green band at the bottom edge, and check your generation figures for a slow decline that matches. Deal with the source where you can: cut back overhanging branches from ground level and keep gutters clear. Do not use a hosepipe from a ladder, do not use a pressure washer, and do not use household cleaners or anything abrasive on the glass.
When should you call a solar installer?
Use a solar installer or a specialist cleaner who works with deionised water and a pole system from the ground and carries the right insurance for work at height. Bring a solar installer specifically if soiling coincides with a step change in output, since that combination suggests something electrical as well, or if the frames and clamps need checking while access is set up.
What will the solar installer actually do?
- Assess whether soiling is actually costing output, or whether the loss is shading or a fault
- Clean with deionised water and a soft brush, from the ground where a pole system reaches
- Avoid pressure washing and detergents that damage the panel's anti-reflective coating
- Inspect frames, clamps and visible cabling while access is in place
- Check the array for cracked or delaminating panels found during cleaning
- Re-check generation after cleaning to confirm how much the soiling was costing
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