One word: trees. Plus, the only thing you're going to get from GIS is a building foot print. There's no data on the slope of the roof. You need to stop thinking urban because contrary to popular belief, most people don't live in an urban area. I can show you two different Google satellite images of my house, one taken in June and one taken in January. The shadows case by the trees are dramatically different. My point is that you can't get an accurate assessment of available sunlight throughout the day on just a computer model alone. Here in Arizona, there are a lot of lawsuits brewing against outfits like SolarCity because they convinced people to lease systems saying the lessee would be making money by selling the excess electricity. That turned out to not just be false but it's actually costing people more than before because of all the hidden fees involved.
As an engineer, my first thought is: Hey, cool, I can cut my electric bill to the bone. I've got a huge roof with southern exposure. That roof shows up great on Google imagery. But then I actually look at it throughout the day and the pine trees cast long shadows that sweep over the whole thing. It's worse in the winter when the sun is lower. My neighbor, on the other hand, just had a system installed. 18 panels. But his house is totally unobstructed because he's on the top of a hill with full southern exposure. Ultimately, solar will never be this magic talisman that makes everybody's energy problems go away. Right now, I'd compare it to the dotcom era where people are willing to believe everything and the equipment providers are looking for any way to market the product because they're making money of the subsidies. Eventually, people will realize that it's not all that and a bag o' chips.