The thing is, even with a coal fired powerplant electricity is dirt cheap. I mean its redicolous the amount of energy it can provide at little cost. Its only expensive because of the taxes that are put on top imo. And if not, its the costs to maintain the grid itself that is most of the price. So even if we get Nuclear Fusion electricity wont be much cheaper unless the grid becomes more decentralized? Ie. power generation on the spot, at home, on demand, and that taxes and so on go to 0 or near 0.

on topic: First thing that comes to mind is some sort of plasma propulsion engine because plasma is everywhere in space