Er, we already have Arc reactors. Even first gen Arc reactors pack three gigawatts of power generation* into a reactor the size of a fist, can store a large amount of energy, and require no fuel, which is just ridiculous already.
*Well, maybe not. The Iron Man movie says the Mark I puts out "three gigajoules per second" and can power "something big for fifteen minutes". So, the actual generating power is probably lower, with the reactor charging a supercapacitor-equivalent that can hold 3GW*15min*60s/min = ~2.7 Terajoules, which can be discharged at a max rate of 3GJ/s for that fifteen minutes of badassery.
*Well, maybe not. The Iron Man movie says the Mark I puts out "three gigajoules per second" and can power "something big for fifteen minutes". So, the actual generating power is probably lower, with the reactor charging a supercapacitor-equivalent that can hold 3GW*15min*60s/min = ~2.7 Terajoules, which can be discharged at a max rate of 3GJ/s for that fifteen minutes of badassery.