According to wikipedia,
alternators are only roughly 50-62% efficient. So, I'd have to disagree that its efficient.
For the rest I'll need to run some calculations. 230W is .3 hp. So, you're talking .6 hp to power a Metro which is probably about as low of a electricity hog of a vehicle as you can get. Any newer vehicle is sure to need more electric power.
From my reading on designing supercharger systems, belt losses are around 1% IIRC. If you're cruising down the road and and require 10hp to maintain speed, the alt is eating .6hp, and the belt is eating .1hp.
I think a much better ($) way to go is forget solar and stick with deep cycle batteries and a charger.