I had an 80W panel on the roof of my van that I used for a few weeks to keep it running when the alt died in it. It worked out ok but wasn't quite right for keeping it going. It was a Astro with a 6.5L diesel so it didn't need power to run, just start and accessories. The problem was after driving the batteries would be weak so I would have to sit and let it recharge. After a while the batteries would be full charged and the rest of the energy the solar panel was generating got wasted. With the second battery setup like I am thinking of, the second battery could go much lower than normal while still charging the car and use the solar power generated much better since that battery will be at full charge less often. So it would run longer without any problems and still have enough power to start it for longer periods of time. Even with the efficiency losses it should still work out ok.
I was looking at this panel
Amazon.com : Renogy® 100W Monocrystalline Bendable Solar Panel : Patio, Lawn & Garden I could probably get 4 of them on a metro. 2 on the hood and 2 on the roof. If they put out half their rated power for 3 hours a day that is plenty of power to run a Metro for the at most 3 hours I normally drive it. The metro only has a 60 amp alt from the factory and has a pretty simple electrical system.
I know I can plug it in at night to charge, there are a few people with Metros that do that and it works out well for them with no solar panels or anything fancy. This setup is more automatic and keeps everything happy by supplying 14.4V to the car while it is running using the step up converters to charge the main battery so the car always starts and runs like normal. It won't be cost effective at all and I figure probably 1500 bucks for the whole setup but it might get me another few mpg without having to mess with the car all the time. It is moderately easy to make an alt kill switch so I could always leave it in the car for when I actually need it.
Ultimately it is hard to get a metro over 70mpg without spending way more money than can be saved. I have a custom smaller cam than an XFi, Aero mods, weight reduction and am currently working on retuning the ECU. I can get upper 60s in the car the way it is now no problem on a tank, but it just cant quite make it to 70mpg. My options are starting to get expensive for whatever I do now. I figure 1500 to get rid of the alternator might get me 95% of my normal driving covered with no problems and pick up maybe 3-5mpg and put me over 70mpg. It isn't about saving money any more it is about getting the highest mileage possible in a car with normal driving and treating it like a regular car.