I have a (very) small panel keeping an old car battery topped up in the Shed. It seems to get plenty in the summer, in its current 'propped up against the wall' arrangement. The Shed is very conveniently arranged in terms on its geographical orientation, with a 'porch' section of the roof sloping to the south.
When I get around to mounting the panel properly, my intention is to have it (more safely) on the Shed roof, facing south and at around a 70° slope, to (slightly?) increase the input in the winter periods, though at the expense of the summer input, which is a loss that I can afford, I think. It gets more input in the summer, in both time and intensity, and has slightly less outgoings. This may also keep it cleaner, although that doesn't seem to have been an issue in the four years that it's been running in its current location.
Most of the Shed roof has a slight slope to the north and, when up there to clean the chimney, I do find quite a few pieces of gravel from the drive - courtesy of the crows, I presume. I do often wonder how high they drop them from?