If you live at a higher latitude it may be more efficient to mount the panels vertically versus horizontally. Of course, if you have the option for a fixed mounting you'd want to angle the panels a bit shallower than (90 - average elevation angle of the Sun) over the year. But, on an office building or tower like structure, mounting the panels on the south facing wall (northern hemisphere) or north facing wall (southern hemisphere) -- the problem is that if another building is built next to the Sun side of the building and it blocks the Sun then you're power generation is significantly reduced.
Imagine a building in the northern hemisphere at, say, a 50N latitude on the north shore of a body of water such that no other building can be built that might obscure the Sun. If that building were, say, 100m wide on the sun facing side and 300m tall it might generate as much as 3MW and over an average day perhaps 16-20MWh per day -- that amount of power/energy might well be more than the building needs. Of course, window area would likely reduce the power/energy generated by a third or more. But, there are solar windows that generate some power while providing visibility through them so the net could be 75% of a full coverage system or, in this example, about 12-15MWh/day which could be enough to fully power the building.
Brian