It will depend if ou use a fixed solar panel system at your home and charge the Volt with it, or want to carry the panel in or on the Volt. Some photovolaic cells can generate up to 2 watts per square inch, so if you want to have them on the Volt, measure your Volt's roof area and multiply it by the power per square inch of the cells you will buy.Impressive! It's nice to see this sort of hands-on engineering in schools. I was also reading earlier this week about a solar-powered car that students built in Australia.
As for the Volt, a solar panel option would be great. If it only adds 3-5 miles of range a week, at least that's something extra.