yeah, the whole roof in panels puts out enough on a sunny day to allow me to drive for about 45 minutes. (summer)
Winter and I probably get enough to run for 20 minutes.
Those little guys probably have current ratings or wattage ratings so you could figure out how much power they have.
Power = voltage * current
(Watts = volts * amps)
If you have 2 watts of solar panel and you have it in direct sun for 3 hours, you have 6 watt - hours
Your car probably requires about 200 watts minimum to run
(source)
6 watt hours divided by 200 watts = 0.03 hours that you could run your car on stored solar energy.
You might not even see gains from disconnecting your alternator. I saw negative MPG changes. Now I have a really expensive battery maintainer on my roof, lol. (unless I figure out how to get it to like running at 12V)
Matt