It's nice to hear about new advancements/concepts in the EV industry http://www.engadget.com/2014/05/08/bmw-designworks-solar-carport/?ncid=rss_truncated
I bet it works better than you think as I count 24 solar panels. I have 28 panels on my house and I estimate the Roadster uses 8 and the house 20 of my 255 watt panels. I figure each panel is worth 3 miles of driving per day in NE Tennessee in my Roadster. So my use is 3 miles X 8 panels X 30 days or 720 miles a month or over 8000 miles/year.
It looks fairly flat, so they are going to loose a lot of the panel rated power production, but it's still probably more than enough for most people. I have only 20 240W pannels, and I produced over 8628kWh last year. That's enough to drive the Model S for 25.9k miles (assuming an average 333Wh/mile).