For most British homes, rarely. Rain does most of the work on a normally pitched roof, and the output gained from cleaning a reasonably clean panel is small.
This is one area where the UK climate genuinely helps.
When cleaning is worth it
- Shallow-pitched roofs, where rain runs off slowly and dirt sits
- Under trees, particularly with sap, leaves or bird droppings
- Near farmland, during ploughing or harvest
- Near a busy road or industrial site
- Coastal properties, where salt builds up
- Flat-mounted panels, which do not self-clean
- Visible soiling, especially bird droppings, which block light completely rather than dimming it
That last point is the one worth acting on. General film across a panel costs a little output. An opaque spot can affect a whole section of the panel, and on a string system it can affect other panels too, for the same reason shading does. See shading.
How often
If none of the above applies, checking every year or two and cleaning when it is visibly dirty is plenty. Somewhere between never and once a year covers most UK homes.
Do not pay for a scheduled cleaning contract unless you have a reason. Solar panel cleaning is a category with a certain amount of unnecessary selling in it.
How to do it safely
The most important point: do not go on the roof Falls from height are the main risk here and it is not worth it for a few per cent of output.
If you can reach from the ground:
- Use a soft brush or a water-fed pole
- Use plain water, or deionised water if you can
- Clean early or late, not in the middle of a hot bright day. Cold water on hot glass risks thermal stress
- Never use abrasives, scrapers, strong detergents or anything that could scratch the glass or damage the anti-reflective coating
- Never use a pressure washer. It can force water past the seals
If you cannot reach from the ground, get somebody with proper access equipment.
What can void the warranty
- Walking on the panels
- Abrasive cleaning, or chemicals the manufacturer prohibits
- Pressure washing
- Damage to the frame seals or the junction box
Check your manufacturer’s cleaning guidance before doing anything. It will be in the handover pack.
Snow
Snow will slide off a pitched roof as it warms. Leave it. Do not attempt to clear it, and do not use hot water on cold glass.
A snowy day in Britain produces very little solar anyway, so there is nothing much to gain.
What else to check while you are looking
A visual check every so often is worth more than cleaning:
- Any panel obviously discoloured or damaged
- Anything loose, or fixings that look wrong
- Cables hanging where they should be clipped
- Birds nesting underneath, which is common and causes real problems. Mesh around the array edge prevents it and is worth fitting if you have pigeons
- Vegetation growing into the array
And look at your monitoring occasionally. A drop in output is much more likely to be a fault than dirt.