@evildictaitor: Well a focused beam doesn't lose power at the square of the distance (think of a laser beam).

I can imagine some system sometime in the future that can lock on and track devices that need charging, sending focused beams of energy to them. I'm not sure what the energy beam would be though, it has to penetrate most objects and be harmless to humans and animals. Obviously if something gets in the way that the beam can't penetrate it would automatically shut off.