The physics of the situation really don't have me sold that it could actually contribute to the range of the vehicle. I see it more as a battery tender that could prevent the "vampire effect" of a sitting battery, or help aide the cooling/management system that helps tend to the battery. depending on the price of such an option I would consider it.
According to Musk at the National Governors Association closing session today (7/15), a solar roof doesn't make sense on a car. After looking at the option carefully, they determined that it can't produce enough charge to have value. If they could create a solar sheet that roles out over the entire car, and if it is left in a place with good sun, you might get 20 miles of charge a day. No solar roof option for the Model 3.
Even with good sun, and a sheet across a car, the insolation would be crap, as none of the surfaces would have the optimal solar tilt and azimuth for more than a few minutes.
It’s the same (efficiency) issue as with fixed roof panels on a house, except even worse, as in case of a car, all the surfaces are curved and convex in addition to being fixed. Convex is pretty much the worst shape in this case.