Is having a small solar panel on the roof of a car possible?

Many of us spend so much time in our cars, going back and forth to work, dropping the kids off at school or just picking up groceries that surely that would be the most obvious thing to be trying to convert to solar power?

When you park your car up outside your place of work it could be charging itself up for 7 or 8 hours which would give you enough charge to get home and maybe even drop the kids off at soccer practice.

Is having a solar power panel on the car possible, or are we going to have to aim for electric cars that we charge up using solar power? Would they still work on cloudy days or would we have to still have gasoline engines in them so they can run even when the batteries are not charged?

I love the idea of cars that use no gas, but it also needs to be practical, I mean there is no point in a car that wont take me further than the end of the driveway or one that costs as much as the house it is sitting in front of.