Any electric heater, even those decades ago, was 100% efficient. They turn electricity watts into heat watts. Where is the energy loss? Light? That also becomes heat. Air movement? Also heat. Loss due to heat allong the cord to the heater? Thats heat too. Unless they are emitting large numbers of neutrinos, all electric heaters are simply resistors that perfectly turn electricity into heat.
Put nearly any electrical device in a box, anything from a television to a cement mixer, and it will raise the temperature of the air in that box by exactly the same amount as the watts it draws from the power source. A 500w television puts out exactly as much heat as a 500w heater.
You can't have a wind turbine which extracts 100% of the kinetic energy from wind, because after it there would be a wall of unmoving air, and the incoming air wouldn't be able to get through the turbine.
Analogously, if electricity is carried with the flow of charge - electrons - around a circuit[1], when you extract 100% of the energy as heat, the electrons stop moving and build up in the heater. So you can take the rest of the wiring away because it's doing nothing and save 50% of your costs. Then, a buildup of charge makes a voltage, and a voltage potential difference can drive a current. Therefore you can get 100% of the power out as heat, save half your money on wiring, and use the growing potential difference to power something else. Electricity makes no sense whatsoever.
OK, so that's troll-physics nonsense, but does extracting all the "energy" stop the electrons moving? If not, why not, what energy isn't being extracted? If so, why doesn't that stop current flowing - isn't "free electrons" part of what makes something a conductor of electricity?
[1] though the energy is carried in the e/m field around the surface, somehow
Put nearly any electrical device in a box, anything from a television to a cement mixer, and it will raise the temperature of the air in that box by exactly the same amount as the watts it draws from the power source. A 500w television puts out exactly as much heat as a 500w heater.