I've never used one but anyway....
One thing I have seen a LOT of on the China-direct sites is that they sell USB phone-charger packs that have a solar panel on them, and with batteries built inside. It has USB jacks to connect the phone or similar items to. To recharge it you can connect it to AC power, 12vdc/car power or just leave it in the sun. Most look about the size of a 7" tablet, and maybe an inch thick at the most.