Electrons. I've set up a solar panel system I can use to recharge people's batteries from AAA up to 12v car batteries. The best things about this is:
1. I'm using those panels to run my computers right now, saving me money.
2. As long as the panels remain intact I can keep trading energy from them, they won't run out of electrons.

It's like being Ben Bernanke, I'll just be able to keep printing money!

Of course, there's also the whole wild edible and medicinal plant stuff.
-Blast