Solar on the road

How hard and/or expensive would it be to make a car out of passive solar material and use the current to charge a battery for accelerating from 0-3 mph? Or 0-10 mph? Just think of all the cars in CA. I did some digging in the US Census site quickfacts.census.gov and estimate there were around 24,000,000 cars in CA in 2006. If we put cheap passive solar on the roof and hood of each car (forget the trunk) and assume 30 sq ft of surface area for each car, we get a whopping 720,000,000 sq ft (about 16,000 acres) of "free" solar collectors! Adding cheap passive solar to all cars in California would probably generate enough power to offset the CO2 emissions of 1 or 2 percent of that state's cars. Anyone want to do the analysis and post it here?