The Japanese company KYOCERA produces some of the most efficient solar cells.
I have a 11inX7in solar panel with solar cells from Kyocera.
http://www.amazon.com/CDT-5w-Solar-Panel-crystalline-charger/dp/B000NP349G/ref=sr_1_1?ie=UTF8&s=hi&qid=1227190441&sr=8-1I have it charging a Absorbed Glass Mat lead acid battery.
http://www.windsun.com/Batteries/Battery_FAQ.htmI have the solar panel mounted on my car.
It charges the AGM battery inside but on a seprate circuit from the car battery or any part of the car.
Every two days I would use the system [solar-AGM battery] to charge my PDA/phone [palm treo].
Every morning, afternoon, and sun-down, I would measure the voltage.
The morning voltage is the most accurate for measure because it has not been affected by power from the solar panel.
The average voltage during the summer was 12.8V.
The average voltage at this time is 12.5V.
My cell phone charges for about 1.5hours at .5A (or 500mA).
So, if I cover my entire car with these solar cells, it still wouldn't amount to much. However, the insignifigant amount of energy it does produce is fine for powering accessories.
It just won't power my car for daily travel.