What dirty solar panels actually cost you in Sydney
Soiling — the film of dust, salt, pollen and bird mess on your panels — typically cuts solar output by about 5% a year, and up to 25% in the dirtiest locations, according to the US National Renewable Energy Laboratory. On a Sydney harbour-side roof, a scheduled clean usually pays for itself in recovered yield.
The tax you never see itemised
Your panels lose output the same way your windows lose the view: gradually, then obviously. Dust settles. Salt drifts in off the harbour. Pollen lands in spring, bird mess all year. None of it is visible from the ground, and your inverter will not send you a note about it. The loss just sits inside your bill, sunny day after sunny day.
The industry calls it soiling, and it is one of the better-measured problems in solar. The US National Renewable Energy Laboratory, which runs a national soiling-monitoring program across hundreds of sites, puts the typical annual energy loss at about 5% — with a range that runs as high as 25% where grime is heavy and rain is rare.
Why rain does not fix it
Rain helps, which is exactly why it misleads. A decent downpour rinses loose dust, and research shows year-round rainfall claws back a couple of percentage points. But rain does not touch the stubborn layer: baked-on bird droppings, the greasy salt film that builds near the coast, and the grime strip along the bottom edge of each panel where water pools and dries.
That bottom-edge strip matters more than it looks. Panels are wired in strings, and a shaded or soiled band across one edge can drag down more than its share of the array. The film you can not see from the street is the reason two identical systems on the same street can produce noticeably different numbers.
Why a garden hose is not the answer
The tempting fix is a hose from the gutter line or a brush on a pole with whatever is under the sink. Both are how panels get damaged. Tap water dries to mineral spotting. Household detergents can react with anti-reflective coatings. And walking a tiled roof to scrub at arm’s length is how people get hurt.
The proper method is unglamorous: a soft brush rated for glass, purified water that dries without residue, no harsh chemicals, no pressure. That is the whole trick. It is careful labour, done at height, finished with a check of every panel before anyone calls it done.
The Sydney pattern: quarterly near the water
How often depends on where the house sits. Inland suburbs with regular rain can stretch intervals out. Close to the harbour or the coast — Mosman, Vaucluse, Bronte, anywhere the morning haze is salt — the film rebuilds fast enough that a quarterly clean is the honest cadence. That is why solar cleaning sits inside the Schmick Plan as scheduled work rather than a one-off.
The maths is simple enough to do on a napkin. Take your system size, your feed-in and usage rates, and shave 5% off annual generation. For most decent-sized Sydney systems that number comfortably clears the cost of keeping the glass clean — before counting the longer panel life that comes from catching lichen and hot-spot grime early.