Rain Alone Doesn't Keep Panels Clean

One of the most common assumptions we hear across Devon, Cornwall, Somerset and Dorset is that because the South West gets plenty of rain, solar panels look after themselves. Unfortunately, that isn't how it works. Rainwater is good at shifting loose dust, but it does very little against the things that actually cut output: baked-on bird droppings, tree resin, lichen spores, agricultural dust, and the thin film of salt that settles on rooftops near the coast.

Our climate makes it worse in some ways. Mild, damp winters encourage moss and lichen growth on roof tiles, and spores wash down onto the glass. Spring brings a heavy pollen load from hedgerows, woodland and farmland. Coastal towns in north Cornwall and south Devon add salt spray to the mix, which dries into a hazy layer that quietly blunts performance. Rain hits that layer, but it doesn't dissolve it.

What Soiling Really Costs a South West System

Solar panels are tested and rated in clean, controlled conditions. In the real world, a dirty array underperforms. Typical figures for moderate soiling sit somewhere between 5% and 15% of lost generation, and heavy soiling — clusters of gull droppings, lichen patches, a slick of farm dust — can push losses well beyond 20%.

Put numbers to it and the picture sharpens:

• A typical 4kW domestic array generating around 3,600kWh a year loses roughly 180kWh to 540kWh at those rates.
• At current import and export tariffs, that can represent anywhere from £60 to £180 a year in electricity you either have to buy back or don't get paid for.
• If you have battery storage, dirty panels also mean less surplus going into the battery for evening use.
• A 50kW commercial roof losing 8% is forfeiting thousands of kilowatt hours annually, plus the associated export and self-consumption benefit.

The effect is rarely uniform. Shading from a single stubborn patch of lichen can drag down an entire string of panels, because output is limited by the weakest link.

The Right Kit — and the Right Techniques

Cleaning solar panels properly is not the same as cleaning a conservatory roof. A few points matter enormously:

• Purified water. Specialists use deionised or reverse-osmosis water, delivered through a water-fed pole. Because the water carries no minerals, it dries without streaks or limescale spots — important, since residue left on glass is itself a shading layer.
• Soft brushes only. Purpose-made bristles lift grime without scratching the anti-reflective coating. Abrasive pads, scouring materials and household sponges will permanently degrade the glass.
• No pressure washers. High-pressure jets can force water past frame seals, stress micro-cracks and, on some systems, void the manufacturer's warranty. A gentle flow and agitation is all that's needed.
• Temperature awareness. Cold water on a midday-hot panel can cause thermal shock. Sensible operators clean early morning or late afternoon, or work on cooler overcast days — which, thankfully, we have plenty of.
• Safe working at height. Roof anchors, harnesses, edge protection and proper insurance are not optional extras. Falls from domestic roofs remain one of the most common serious accidents in the trade.

Maintenance Checks That Come With the Clean

A good clean is also an inspection. While someone is up there with a clear view of every panel, they can spot problems long before they become expensive:

• Cracked or chipped glass, snail trails and delamination
• Discoloured cells or visible hot spots, often caused by a failing diode or a persistent patch of shade
• Loose or corroded MC4 connectors, damaged cable clips and rodent-chewed wiring
• Corrosion on mounting rails and loosened bolts after winter storms
• Debris trapped behind panels, which can hold moisture against the frame

Small faults caught early are cheap to fix. A failed junction box or a hot spot left to develop can take a panel out of service entirely, and replacements are rarely a like-for-like match.

Planning a Cleaning Schedule That Pays

Frequency depends on where you are and what surrounds you:

• Inland towns and villages: twice a year is usually sensible — late spring after the pollen peak, and early autumn after leaf fall.
• Coastal properties: three or four visits, with an extra clean after a spell of onshore gales.
• Farm and rural settings: spring, midsummer and post-harvest, when dust and slurry particles are airborne.
• Commercial arrays: quarterly is a common baseline, with the schedule adjusted using your monitoring data.

Timing brings its own returns. A spring clean captures the whole high-generation summer period. A pre-winter clean matters less for output, but removes moss and debris that would otherwise trap damp against frames through the wettest months.

Getting the Best Value From a Specialist

Ask about purified water, insurance, working-at-height competence, and whether a visual inspection report is included. If you have monitoring, note your output before and after — a well-executed clean on a genuinely soiled array often shows up within days.

For most South West homes and businesses, professional cleaning is not a luxury. It is a modest, predictable cost that protects a significant investment, supports steady generation, and avoids the sort of unnoticed damage that turns a simple clean into a repair bill.