The "big loop" concept is fundamental: the power lines we use to move energy around are all closed loops, so, even with DC, the mass of electrons that flow in one direction is the same as the mass that flows in the opposite direction. There is no transfer of mass.
This is different from ectrostatic generators: there, AFAIK, there should be an actual transfer of mass. https://en.wikipedia.org/wiki/Van_de_Graaff_generator