With these maps (from NREL.gov) you can see where it is best to have solar installations, but it becomes more and more viable as the cost goes down and energy prices go up! Just remember that energy prices have never gone down, so right now Renewable Energy systems seem only viable with incentives or rebates, but in the long run they will pay for them selves in a crescendo of payback. More on calculating solar ROI in the future.
Direct Normal Solar Radiation (Two-Axis Tracking Concentrator)
PV Solar Radiation, Flat Plate, Facing South, Latitude Angle