There's a lot more to it than just area. The Tesla tiles have to follow the architectural pitch (slope) of the roof, so the solar collector shingles may not be (and probably won't be) at the optimal angle for most efficient collection. Ideally the collector is at 90 degrees to the sun, and in an active system the collector panels move to track the sun.
Then they have to figure in the percentage of sunlight over the course of a year. Not just daylight hours, but how many days are rainy, how many days are heavily overcast, and how many days actually get bright sunlight.
Going back to the original article, I noticed that the payback period is actually more than 30 years -- but the system is only guaranteed to keep water out of your house for 30 years. Which means there's a good chance you'll be paying to replace it before it has even paid for itself. I'm probably not going to live to 104, so I don't see a Tesla solar roof in my future.