What you are actually seeing on the glass

Rainwater is not the clean rinse most people imagine. As a drop falls and then runs down a panel, it collects whatever is sitting on the surface: fine dust, pollen, tree sap, soot from a wood burner, and dissolved minerals picked up from the atmosphere. When the water evaporates, those solids stay where they landed. A single shower can leave a faint outline; a fortnight of small showers can leave a web of pale trails across the glass.

The pattern matters. Water that beads and rolls leaves little behind. Water that sits, spread thin across the panel, dries in place and deposits everything it was carrying. That is why two identical roofs a mile apart can look completely different after the same weather. It is not the rain that stains the glass — it is what the rain was holding when it dried.

Why the South West tends to see more of it

Our corner of the country has a particular mix working against clear glass. Prevailing westerlies carry sea salt inland, so properties within a few miles of the coast often find a fine, slightly sticky film that ordinary rain will not shift. Inland, agricultural dust from dairy yards, arable fields and slurry spreading drifts onto roofs, and moorland peat and heather pollen add to the load in spring and early summer.

We also get the occasional Saharan dust event, when a southerly airflow drops a reddish-brown layer across Devon, Cornwall, Somerset and Dorset overnight. That deposit is mineral-rich and dries into stubborn, cement-like marks if it is left through a dry spell.

Hard water is less universal here than in the South East, but it is not absent. Parts of Wiltshire, Somerset and Dorset sit on chalk and limestone, and where a supply is mineral-heavy the residue on glass is a chalky white rather than a grey smear. Very soft, peaty water from upland sources behaves differently again — it leaves dark, faintly acidic films that can be just as visible.

The frame and edge problem

Most streaking starts at the edges. The aluminium frame collects grit, moss fragments and lichen in its channel, and the underside of the top edge holds a surprising amount of dirt. When rain sheets across the panel, it washes that material down and concentrates it along the lower edge and the gaps between panels. The result is a series of dark vertical lines that reappear within days of any cleaning that ignores the frame.

Bird droppings, and the smaller but relentless deposits left by roosting birds, behave in the same way. They are acidic, they hold moisture, and rainwater spreads them into a long streak that dries hard against the glass. Seagulls along the coast are a genuine maintenance factor rather than a joke.

What the streaks cost you in output

Soiling is a real but often overstated loss. For most domestic arrays in the South West, a typical year of dirt costs somewhere between three and six per cent of generation, with the worst weeks after a long dry spell or a dust event pushing higher. On a commercial roof the numbers add up: a system generating 50,000 kWh a year can lose a couple of thousand kilowatt hours to grime that nobody notices from ground level.

There is a second, more serious cost. In a series string, the weakest panel drags the whole string down, so one heavily soiled module in a row of twelve can take a disproportionate bite out of the day's output. And if mineral deposits are left to cycle through wetting and drying over months, they can bond to the anti-reflective coating and etch it. That damage is permanent — no amount of scrubbing brings back a coating that has been chemically marked.

Watch for a slow, steady decline in your inverter readings. Sudden drops usually mean a fault. Gradual ones usually mean dirt.

Choosing a cleaning method that works

The right approach is gentler than most people expect.

• Use pure water. Deionised or reverse-osmosis water with a total dissolved solids reading below ten parts per million dries without leaving anything behind. Tap water, even soft tap water, will spot.
• Use a soft bristle brush on a telescopic pole, working from the ridge downwards in overlapping sections so dirty water never runs over clean glass.
• Avoid washing-up liquid, vinegar, bleach and abrasive pads. Detergents leave a film that attracts dust; acids attack coatings; scourers scratch.
• Clean when the glass is cool. Early morning or late afternoon. Warm glass dries water too quickly and bakes on whatever is dissolved in it.
• Keep pressure washers away from the panel edges. High-pressure water forced past the frame seals is one of the most common causes of moisture ingress and later faults.
• Rinse the frame first, then the glass, then give the whole array a final pure-water rinse so nothing is left to dry in place.

If your roof is steep, fragile, or more than a storey up, work from the ground with a pole or bring in someone who does this regularly. Falls from domestic roofs are far more common than most homeowners expect, and a slipped foot costs far more than a season of dirt.

A simple year-round routine

Two cleans a year suits most homes: one in late spring after the pollen and Saharan dust season, and one in early autumn before the low sun and short days make every stray mark count. Coastal properties and those near farmland often benefit from a third clean.

Between cleans, clear leaves and moss from gutters above the array, trim back overhanging branches, and check that no new shading has appeared from a growing hedge or a new satellite dish. Once a year, look over the mounting fixings, the cable clips and any exposed conduit. Panels that stay clean and properly fixed will quietly out-earn identical ones left to fend for themselves — and they will look considerably better doing it.