Ordinarily, solar panels make the most sense on the roof of a building. This is often where you can harvest the most solar energy. But sometimes it can make sense to install them vertically, like in the case of the above building in Scuol, Switzerland. I am by no means an expert on solar, but I am told that this can make sense in snowy climates (where rooftop panels are likely to get covered) and in northern locations where the lower angle of the sun means a vertical position actually catches the sun more effectively (especially during the winter months). It could also be the case that you just don't have any available roof space. Either way, you have to look at and model out the entire year when it comes to solar.