>There's no such thing as an efficient space heater. They are all the same efficiency: 100%, because all the energy goes to it's intended use, which is heat.

That's not true. Or at least, it's only true for resistive heaters.

Electric power can also be used to drive heat pumpts, which can turn 1 electricity into 3-5 units of heat (depending on the outside temperature).

Stealing heat from the outside is more efficient than creating it.