The Average angle of the sun to the Ground is 45 degrees, The...

The Average angle of the sun to the Ground is 45 degrees, The Average angle of the sun to a wall is the same 45 degrees.

So Walls would get almost the same sun Angle as the roof for about the same amount of light per day on well chosen walls.

There is a hundred times as much available wall space for Solar glass Windows than there is for Roof space on Highrise buildings.

Its collecting solar power from All the City Glass, its a massive amount of renuable energy available to be Harvested.