Solar panels work best when facing south and inclined towards the sun. This leads installers to park them in gardens (illustration, right) like vehicles. This is not imaginative arrangement. As solar panels become cheaper, maximising their energy gain will become less important and we can expect to see their incorporation in building design. The obvious place for a solar panel is on a south-facing pitched roof. Except for nostalgia, there seem to be few other reasons for tiles to continue in use. But roof surfaces will be needed for other purposes and in the longer term it seems more appropriate to fix solar panels on walls than on roofs. Solar panels can work as photo-voltaic cells, to generate electricity, or as systems for heating hot water (typically a black radiator behind a sheet of glass).
Photo-voltaic (PV) cells generate electricity
Solar panel in garden
Solar roof from Wikipedia