ELI5. If an electric heater is not 100% efficient. Where is it losing its energy?

Question

Ok I get that some heater are more efficient at heating an area, person heater over space heater etc.

But sometimes
The efficiency gets quoted. And other than losing energy at heat (which seems fine) how else do they lose energy?

In: 30

Answer 1

SO what you are telling me is that I should make a “heater”, that is just a cord, that dissipates heat.

Me a 5 year old: ” It’s 100% effecient ”
CFO: “Sir we can’t market this it’s going to catch the things it touches on fire.”
Me: “100% of the energy is heat. It’s 100% efficient.”