Dirty solar panels lose 10–25% of their output. In the UK, rain keeps most panels reasonably clean, but it doesn't remove bird fouling, moss, lichen, or the sticky residue that builds up near motorways or farmland. Here's how to clean safely and effectively.
When do panels actually need cleaning?
Panels need cleaning when: - Bird fouling is visible on 2+ panels (even one heavily fouled panel can reduce whole-string output) - Moss or lichen growth is visible (especially on north-facing sections of split arrays) - Output has dropped 10%+ on a clear day vs the same time last year (check your monitoring app) - The system is over 12 months old and you've never cleaned it
Panels usually do NOT need cleaning after: - Normal rainfall (light dust washes off) - One bird dropping (unless directly on a panel cell — then clean it)
Safe cleaning method
What you need: - Telescopic soft-brush pole (3–4 m minimum for a typical 2-storey UK house) - De-ionised water supply (a basic RO filter system costs £50–£80; tap water leaves limescale streaks) - Bucket or pump spray bottle
Method: 1. Clean in the morning on a cool day — thermal shock from cold water on hot panels can (rarely) cause micro-cracks 2. Pre-wet panels with de-ionised water 3. Soft brush in gentle circular strokes — no pressure, no abrasive pads 4. Rinse with clean de-ionised water 5. Let air dry — no squeegee
Never use: - Pressure washer above 300 psi (delaminate panel edges) - Abrasive cloths or steel wool - Bleach, acids, or commercial glass cleaners (attack anti-reflective coating) - Solvents (acetone, alcohol) on cell surfaces
Bird fouling: a harder problem
Simple bird droppings can be soaked with de-ionised water for 15–20 minutes, then gently wiped off. Heavy encrusted fouling requires longer soaking and sometimes a second pass.
If you have recurring heavy bird fouling — especially from gulls, starlings, or pigeons roosting under the panels — the right solution is bird mesh (aluminium mesh skirting installed around the panel perimeter). A single clean fixes the current problem; bird mesh prevents it recurring for 15+ years. We install both.
Moss and lichen: a slow damage risk
Moss and lichen grow slowly but their root structures can penetrate panel edges and cause long-term damage. A biocide wash (sodium hypochlorite diluted correctly) kills the growth — let it sit for 15 minutes, then rinse. A second pass with brush usually needed.
Lichen on the glass surface is harmless. Lichen under the panel frame is a structural concern — call a professional.
DIY vs professional clean
DIY is fine if: - Single-storey or bungalow - You can safely access from ground with a pole - No moss or lichen present - Light dust only
Call a professional if: - Two-storey or higher (fall risk) - Bird fouling is heavy - Moss or lichen is present - You want a health check alongside the clean (thermal imaging detects failed cells invisible to the naked eye)
How often
Once a year is the UK standard for most locations. Near the coast, near trees, or near agricultural land: twice a year. Our annual maintenance plan includes two cleaning visits plus a system health check for £149–£249/year.