Do solar panels need cleaning? When it is worth paying for

Why most panels need very little

Solar panels have smooth, hard glass fronts and sit at an angle, so when it rains the water runs off and takes most loose dust with it. On a typical pitched roof in Britain that happens often enough that many arrays never need a professional clean at all.

The angle is what makes the difference. On a steep roof water runs straight off. Below roughly fifteen degrees it lingers, dirt dries where it settled, and a dark band builds up along the lower edge of each panel where the water last sat. Flat roof arrays mounted at a shallow tilt are the ones most likely to need regular attention.

When cleaning is worth it

• Bird droppings over the cells. Rain rarely shifts them, and a covered cell does more harm than its size suggests, because shading part of a panel can drag down the output of the whole string it is wired into.
• Trees nearby, dropping sap, leaves and seed onto the glass and keeping it damp enough for moss and lichen to start along the frame edges.
• Farmland, building sites and quarries, which coat panels in fine dust through dry spells.
• Main roads and railways, which leave a greasy traffic film rain does not remove.
• The coast, where salt spray dries to a film.
• A shallow pitch, for the reason above.

Check your own figures first

The cheapest way to decide is to look at what the system is producing. Most inverters show total generation on a display or in an app, and many households also have a generation meter. Compare this month with the same month last year rather than with last month, because the season makes far more difference than dirt does.

If generation on sunny days is noticeably down year on year and the panels look dirty, a clean is likely to help. If output has dropped to nothing, or the inverter is showing a fault light, the problem is electrical rather than dirt, and it is a job for the installer, not a cleaner.

Can you clean them yourself?

Only if you can reach them safely from the ground, as you can with a ground-mounted array or a low garage roof. Do not climb onto the roof and never stand or kneel on panels: they are not built to take a person's weight, and the damage it causes is not always visible.

From the ground, use a soft brush on an extension pole and plain water, ideally purified so it dries without spots. Leave out detergents, abrasive pads, scrapers and pressure washers, all of which manufacturers' cleaning instructions commonly warn against, and check your own panels' manual before doing anything. Clean early in the morning or in the evening, because water on hot glass in full sun dries before it can be rinsed off and leaves marks.

What a professional clean involves

Most domestic solar cleaning is done the same way as upper floor window cleaning: purified water fed up a carbon fibre pole to a soft brush, worked from the ground or from a safe position nearby. Nobody should need to walk on the array. A careful cleaner will also look at the frame edges and any gap underneath where birds could get in.

It costs about £5 to £12 a panel, with a minimum charge of £60 to £80 that most domestic jobs fall within. A twelve-panel array usually comes to £70 to £145. Bird proofing mesh fitted around the edge of the array is a separate job at £250 to £600.

How often, and when

Where cleaning is needed at all, once a year suits most domestic arrays, and every six months suits shallow pitches, arrays under trees and roofs that gulls or pigeons favour.

Spring is the usual time, so the glass is clear for the long days that produce most of the year's generation. A winter clean recovers very little, because winter generation is low to begin with.

Pigeons under the panels

The gap between an array and the roof is sheltered and warm, and pigeons nest there readily. The result is noise, droppings across the panels and down into the gutters, and a build-up of nest material that can hold water against the roof.

Mesh fixed around the edge of the array keeps birds out without drilling into the panels. Active nests of most wild birds are protected by law while they are being built or used, so proofing is normally done outside the nesting season, or after a check that no nest is in use. Clearing the gutters at the same visit is sensible, as they usually need it.