Panels mounted flat against a wall are not a compromised roof array, they are a different product with a different output curve. A vertical panel loses a good deal of its annual yield against a pitched south facing roof, and gives some of it back in the months when a house actually needs it, because a low winter sun strikes a vertical surface much more squarely than a shallow roof. Whether that trade is worth making depends on why the roof is not available, which is the question to settle before comparing any quotes.
What a vertical array gives up, and what it gets back
Over a year, a wall mounted panel in Britain generates meaningfully less than the same panel on a well oriented pitched roof, and the shortfall is largest in summer when the sun is high and the wall is edge on to it. The shape of the loss is what matters: output is flatter across the year and the winter months hold up far better, because in December the sun is low enough to meet a vertical surface almost head on. For a household whose demand peaks in winter, a flatter curve is worth more than the headline annual figure suggests, and for one that is out all day in summer it is worth less.
Which wall, and what the fixings have to reach
South is the obvious answer and east or west walls are not pointless, because a vertical east wall catches morning sun that a roof would not see until later. What decides feasibility more often is the wall itself. The fixings have to reach masonry or a structural frame rather than render, insulation or a cavity leaf that cannot carry the load, and a wall mounted array puts a very different set of forces into the building from a roof one, wind uplift especially. That is a structural question and belongs to somebody who will put their name to it, not to the installer's sales visit.
When a wall is actually the right answer
Three cases come up repeatedly. The roof is the wrong shape, too shaded or already full. The roof is not yours to use, which is common in flats and leasehold. Or the roof cannot take the weight or the disturbance, which arises on old coverings and on buildings where any roof work triggers something expensive. In all three the honest comparison is not wall against roof, it is wall against ground mount and against not doing it at all, since a ground mounted frame in a garden usually beats a wall on yield if there is room for one.
What to ask before a wall mounted quote is comparable
Ask what annual figure the quote assumes and what orientation and tilt it was modelled at, because a vertical array modelled as a roof one is simply wrong. Ask what the fixings are going into and who has checked the wall can take it. Ask whether the inverter is being sized for the flatter curve rather than for a roof peak. And ask how the panels will be reached for cleaning and repair, because a wall array high on a gable is no easier to get at than a roof one and often harder.
Questions people ask about vertical solar panels
Do vertical solar panels work in the UK?
Yes, with a lower annual yield than a well oriented pitched roof and a noticeably flatter output across the year. The winter months hold up better than most people expect, because the sun is low enough then to strike a vertical surface almost squarely, and the summer months are where the loss sits.
Can solar panels be mounted on a wall?
They can, and the constraint is usually the wall rather than the panel. The fixings have to reach masonry or a structural frame rather than render or an outer leaf that cannot carry the load, and wind uplift on a vertical array is a structural question that should be answered by somebody accountable for the answer.
Are wall mounted solar panels cheaper than a roof array?
Not usually. The panels cost the same, the mounting system is different rather than simpler, and access can be just as awkward. A wall array is chosen because the roof is unavailable, shaded, full or not yours, not because it saves money.
Is a wall or a ground mount better if the roof is unusable?
A ground mounted frame generally beats a wall on annual yield, because it can be pitched and oriented freely, so where there is garden to spare it is worth pricing both. A wall wins where there is no ground, where the ground is overlooked or shaded, or where a frame in the garden would need permission a wall array would not.