A rooftop array furred green along its lower rail is a modern twist on an old anxiety: the gardener who shrugs at moss on a wall frets the moment it appears on something that cost several thousand pounds. Most of the time the moss is doing far less than it looks. There is one real case where it quietly eats into your generation, though, and knowing which situation you have saves both needless scrubbing and needless loss.
Where moss actually settles
It rarely takes to the glass itself. A panel's face is smooth, steeply pitched on most roofs, and bakes dry within minutes once the sun is on it, so spores get almost no purchase. What moss colonises is the frame: the aluminium edge, the rubber seal, and above all the shaded lower lip where rainwater and grit collect and sit. Low-tilt and ground-mounted arrays are the ones that suffer, because a shallow angle lets water pool and dirt bank up along the bottom rather than run off. The same panels on a steep roof usually stay clean for years, which is why my own pitched-roof set has never needed more than a rinse while a neighbour's low garden array greens over every winter.
When it costs you power
Two different things are going on. Plain soiling, a general film of dust and moss across the face, trims output by a small amount, often only a few per cent, and a good spell of rain washes most of that away for free. The one that bites is shading. A lip of moss and muck along the bottom edge shades the lowest row of cells, and because the cells in a panel are wired in a series string, shading part of that string drags the whole panel's output down out of all proportion to the area covered. Bypass diodes soften the blow without cancelling it. If a low-tilt array's summer generation has drifted downward year on year with no other explanation, a shaded, mossy bottom edge belongs high on your list of suspects, and your inverter's own figures are where to catch it.
Cleaning it off without doing harm
Gentle wins every time here. Still, overcast conditions or an early morning are best, since cold water meeting sun-hot glass can crack a panel and also dries to streaks. A soft brush, or a soft-bristled telescopic wash brush, with plain water (rainwater if your mains supply is hard) lifts moss and soiling without scratching the anti-reflective coating. Keep the pressure washer firmly in the shed; its jet forces water past the panel seals and can lift the laminate, and it is a well-known way to void a warranty. The harsh path-and-patio moss killers have no business here either, since they attack seals and coatings alike. Should you want anything beyond water, a very dilute soap solution, thoroughly rinsed, is as far as it is wise to go.
Doing it safely, or paying someone
This is the point where I hand the job over rather than talk it up. Panels live on roofs and carry electricity, wet glass is slick underfoot, and a live array cannot simply be switched off at a socket. Isolate the system by the manufacturer's instructions before you touch it. Never stand on the modules, since your weight can crack the cells hidden beneath the surface. If the array sits anywhere you cannot reach safely from the ground with a pole, hand it to a professional cleaner or your installer without a second thought. The gain from a spotless panel runs to a few per cent of output; it is nowhere near worth a fall from a roof.
Keeping it from coming back
Prevention asks less of you than cleaning does. Clearing the gutters and any leaf litter above the array stops the steady trickle of debris that feeds the moss, and cutting back branches that overhang the panels removes both the shade and the bird droppings that come with a favoured perch. If you are still choosing a ground mount, a steeper tilt keeps the lower edge draining and slows any green from ever taking hold. The moss creeping over the roof tiles around the panels is a separate matter, dealt with just as it is on any roof; both moss on roofs and removing moss without wrecking the surface cover that ground, and the gentle-first rule carries straight across to them.