Why a Dirty Panel Is a Costly Panel

Solar panels work by letting light reach the cells beneath the glass. Every speck of dust, pollen or bird dropping blocks some of that light, and blocked light is simply electricity you never generate. It rarely looks dramatic — most soiled panels still perform — but the losses quietly add up, especially on shallow-pitched roofs where rain does less of the cleaning for you.

Rain helps, but it is not a maintenance plan. In the South West we get plenty of it, yet rain beads around dirt and dries off in the sun, leaving behind the very film that causes the problem. Panels that look fine from the ground can still be carrying grime that costs you a few per cent of your annual output.

What Builds Up on Panels Across the South West

Our corner of the country is unusually good at dirtying solar glass. Depending on where you live or trade, expect a mix of the following:

• Coastal salt spray — properties near the Cornish, Devon and Somerset coasts pick up a fine salty film that dulls the glass and is surprisingly stubborn.
• Pollen and agricultural dust — the Somerset Levels, North Devon dairy country and the arable belts of Dorset and Wiltshire produce clouds of pollen each spring.
• Bird droppings — gulls in coastal towns and pigeons on town-centre and industrial roofs are the worst offenders, because droppings block light in concentrated patches.
• Tree sap, lichen and moss — common around wooded valleys, Exmoor, Dartmoor's fringes and older gardens with mature trees.
• Saharan dust — occasional airborne dust events leave a reddish haze across the region, usually followed by spotty rain marks.

Add hard water in some areas, which leaves mineral spotting where rain dries on warm glass, and you can see why a routine matters.

What Clean Panels Are Actually Worth

General grime typically costs between three and six per cent of output, with coastal, rural and heavily treed locations sitting at the upper end. Heavy soiling — droppings, moss, a full autumn leaf mulch — can be considerably worse, and uneven soiling is more damaging than an even film because it stresses individual cells.

Put numbers on it. A typical 4 kWp home system in Exeter or Bristol might generate around 3,600 to 4,000 kWh in a good year. Losing five per cent of that is roughly 180 to 200 kWh. At today's unit rates that is in the region of £45 to £70 a year, and more if you are also losing export payments. For a business with a 50 kWp roof, the same percentage runs into hundreds or thousands of pounds over a year, which is why commercial arrays are usually put on a scheduled clean rather than an occasional one.

There is a diagnostic benefit too. Clean panels give you a fair baseline, so a dip in generation points to a fault rather than dirt.

Cleaning Your Panels Safely

If your array is on a garage or single-storey roof you can reach safely from the ground with a pole, DIY cleaning is perfectly reasonable. A few rules keep it effective and safe:

• Work early morning or late evening, when panels are cool. Cold water on hot glass risks thermal stress, and water evaporates before you can wipe it, leaving streaks.
• Use a soft brush on a telescopic pole and plenty of water. Filtered or deionised water dries without leaving mineral spots.
• Skip the pressure washer, abrasive pads and household detergents. They damage anti-reflective coatings and can affect warranties.
• Never walk on the roof. South West slate and tile roofs are slippery when wet and unforgiving when not.
• Leave the electrics alone. No opening junction boxes, no hosing down an inverter, no fiddling with isolators.
• Stay off ladders in wind or after frost, and keep poles clear of overhead cables, particularly on farm and commercial sites.

When to Call in a Professional

Many South West homes, and most commercial buildings, are best served by a specialist visit. If your panels are more than one storey up, set on a steep or fragile roof, crowded with dormers and chimneys, or regularly coated in gull mess, professional cleaning pays for itself in safety and results. Commercial roofs bring their own needs: access planning, method statements, and working around shift patterns so cleaning does not clash with operations below.

Professionals also bring the right kit — soft-water systems, extended-reach poles, and the experience to spot problems while they are up there. A thorough visit should include checking for cracked glass, hot spots, loose cabling, damaged mounting rails and signs of water ingress, plus a look at inverter readings. Cleaning and inspection together give a far better return than either on its own.

A Simple Maintenance Routine for the Year

You do not need an elaborate schedule. Four touchpoints cover most of what the South West throws at your array:

• Early spring: clear winter grime and storm debris before the pollen season begins.
• Late spring or early summer: remove the pollen layer once trees and grass have finished.
• Autumn: clear leaves and sap before the low-sun months, when every kilowatt matters most.
• After winter storms: check for damage, debris and fresh soiling, and review your monitoring data.

Between visits, glance at your generation figures monthly and compare them with the same month last year. A sudden unexplained drop usually means a fault rather than dirt, but a slow drift downwards over a year or two points squarely at soiling. Keep nearby trees trimmed, move bird feeders away from the roof, and consider mesh guards if pigeons have taken an interest. A little routine attention, and your panels will quietly do what you paid them to do.