Capacitor for complete electrical system not just air?

A DC cap like those used on big car stereos, works rather differently than an AC cap. Like Chris said, with 120v AC it's about altering phase. The big 12vdc caps used with car stereos do indeed build charge up to whatever voltage the alternator is delivering and when the voltage sags due to say, a big bass hit on a 1000w stereo, the cap dumps it's energy into the circuit helping hold up the voltage. (and by 1000w, I mean a real 1000w amplifier, not a Walmart 1000w amp)