The yield of electric power strongly depends on the face direction and the slope of the roof. Plywood or wood plate roofs with 15 to 40 degree and face toward to south, south-west and south-east are prefereable to install SolarShingles to achieve a reasonable power yield.
The yield of electric power varies also with the location of your building. In Ontario, 11 KWh ~ 14.5KWh is roughly estimated to be yearly harvested from a square foot of roof.