I'm not sure what you're trying to do, as far as location constraints, etc. but 240 watts is a TINY load. Hell if there's a stream nearby you could run way more than that off of a car alternator and a small water wheel. Most car alt. are over 100 amps @12V or 1200 watts. Solar is pricey, but I've seen solar battery chargers for cars that are like $20. It may take a few of those, but it won't be as much $$ as a good generator.