Just read about Google's new mapping project called “Project Sunroof”.
Not all the US has been mapped yet, but there are a number of locations around Boston MA.
The article, from Ars Technica, says, "it uses its aerial maps database, does 3D modeling of the rooftop, takes into account any shadows cast by trees or nearby structures, and then runs that against “all possible sun positions over the course of a year,” as well as “historical cloud and temperature patterns that might affect solar energy production.”
Now there's some spatial analysis for you.
Know of any real world, successful, and affordable, Solar power installations around where you are?