We've got a 40 watt 12V monocrystalline panel hooked up to a solar charge controller, which in turn is connected to the battery. Ours is a Manson charge controller that is very good. It has three stages of charging (bulk, float and absorbtion) plus a number of additional features. I found the best bargains on eBay, solar panel from Canada, controller from Australia! I wouldn't recommend less than 40 watts, but it depends on how much your battery is used.
Check out this forum, quite a bit of good info/advice http://www.popupportal.com/index.php?board=51.0
This is what I use: http://www.amazon.com/Sunforce-7-Amp-Charge-Controller/dp/B0006JO0XI/ref=sr_1_3?ie=UTF8&s=automotive&qid=1235100570&sr=8-3 Cheap, reliable, and works great. I run 45 wattts of solar panels on my roof, next year I'll get an 85 watt and a 10 Amp solar charger for it. JJ
