Why hard water marks appear on solar glass in the South West

Water hardness varies enormously across our region. Supplies drawn from the chalk and limestone of Wiltshire, Somerset, Dorset and parts of Bristol and Gloucestershire carry far more dissolved calcium and magnesium than the soft, granite-fed supplies of Cornwall and much of Devon. Add coastal salt spray, agricultural dust, tree pollen and the occasional fall of Saharan dust, and your panels are dealing with a lot of airborne material.

When a hose, a sprinkler or even a light shower wets the glass and the water then evaporates, those dissolved minerals stay behind. The first few drying cycles leave a faint film. Repeat that over months and you get hard, chalky scale bonded to the surface. Ordinary rain will not shift it, because clean rainwater cannot redissolve what it left there in the first place. This is why a panel that looks "just rained on" can still be noticeably duller than it was two summers ago.

What the spots actually do to your yield

Sunlight has to pass through the glass before it reaches the silicon underneath. Every mineral speck scatters a small amount of that incoming light away from the cell, and a cloudy, scattered film across a whole array is worse than the sum of its individual dots. Light soiling typically costs somewhere between 2% and 5% of generation. Heavier scale, salt crusting or bird droppings can push that considerably higher.

The numbers add up quickly. A 4 kWp array in the South West might generate around 3,800 kWh in a good year. A steady 5% loss is roughly 190 kWh gone — and much of it disappears during the long, bright days of late spring and early summer, when every unit is worth the most.

Spotting also matters more than surface area suggests. Because panels are wired in strings, a heavily soiled cell or a dense band of scale along the bottom edge can drag down the performance of the whole group. Shading losses are rarely proportional to how much of the panel is covered.

Recognising the problem early

You do not need to climb anything to check. Stand with the sun behind you and look at the array from an upstairs window or across the garden.

• A dull, milky or bloom-like appearance instead of crisp reflections.
• White or chalky rings and tide-lines, especially along the bottom edge of each panel.
• Streaks running down from vents, gutter outlets, aerial cables or satellite dishes.
• Bird droppings, lichen fragments or moss blown from overhanging trees.
• Monitoring data that dips on clear days compared with the same week last year.

If one corner looks noticeably clearer than the rest — often where a bird has washed it clean — that contrast is a fair sign of what the whole array is losing.

Cleaning safely and correctly

Work on cool glass, early morning or in the evening. Cleaning hot panels in full sun creates two problems at once: rapid evaporation leaves fresh marks, and cold water on hot glass risks thermal stress.

• Never use tap water for the final rinse in a hard water area, or you simply re-deposit the minerals you have just removed. Use purified or deionised water, ideally through a water-fed pole with resin filtration.
• Use a soft bristle brush or microfibre pad. Avoid scouring pads, cream cleaners and washing-up liquid, which leave a residue that streaks and attracts more dust.
• Work from the top down, then rinse or squeegee in one pass to prevent drying lines.
• Stay off the roof where you can. A telescopic pole from the ground or a stable platform is safer than ladders, particularly near rooflights and gutters.
• Do not pressure-wash the panels or the roof beneath them. High pressure forces water past seals, damages frames and loosens tiles.
• Follow your installer's guidance before working near wiring. Panels generate DC whenever there is daylight and cannot simply be switched off at the array.

Dealing with stubborn scale

For a light film, warm water with a few drops of a mild, residue-free soap followed by a purified rinse is usually enough. For bonded scale, use a descaler formulated specifically for solar glass. Spot-test on a discreet corner first, dilute exactly as directed, do not let it dry on the glass, and rinse thoroughly afterwards. Never use brick acid, kettle descalers or any product containing hydrofluoric acid, and never reach for an abrasive pad.

One caution worth repeating: many modern panels carry a factory anti-soiling or hydrophobic coating. Aggressive chemicals or abrasives can strip it, leaving the glass permanently easier to soil — and in some cases affecting your warranty. If scale will not lift with careful work, a specialist with deionised water-fed poles, pole-mounted cameras and proper insurance is money well spent.

A practical maintenance rhythm for the South West

There is no single correct schedule, but geography gives you a sensible starting point. Coastal properties should plan on a clean every four to six months, because salt spray bonds tightly and reappears quickly. Inland, tree-lined or rural sites usually manage well with six to twelve months, though pollen season from March to June and harvest dust in late summer can both leave a film worth removing sooner. After a long dry spell, a quick look is always worthwhile.

Pair cleaning with the jobs that keep the rest of the system healthy: clear gutters, trim branches that will soon shade the array, check for cracked or slipped tiles, and review your inverter and monitoring figures. Spring, before the strongest generation months, and early autumn, after a dry summer, are the two natural slots for most households.

For businesses — farms, warehouses, hotels and offices — a quarterly contract with a simple log of dates and conditions is usually the easiest way to protect a large investment. The cost of cleaning is modest set against the energy regained over the following years, and clearer glass means every bright day in the South West works that bit harder for you.