The sun only puts out something like 1500w per square meter onto the surface of the Earth in perfect conditions. Unless we figure out how to generate more solar power than hits the panels, they'll never work on an average car, even in perfect conditions.
Assume an equivalent to a 200W panel on top (though I'd consider it fortunate if someone could pull off even 100W). Five hours in direct sunlight in optimal conditions will give you a kilowatt hour (kWh). That will carry our '11 Nissan Leaf four (yes, the number 4) whole miles...after five hours in the sun. If you live near the equator, and within a couple miles of your workplace, then a Sonos vehicle might be for you! If you live in the Pacific Northwest and don't live next door to work, can I interest you in a lightly used 2011 Nissan Leaf?
Sonos seems to have even gone all-in on the solar thing. "Solar instead of paint", "powered by the sun", which just says to me, "don't look really hard at the details of our offering. Look! Solar panels!"