It really doesn't matter what the specific method of heating water is, it really all comes down to energy going in (as electricity through the cord) and energy coming out (as a certain volume of water heated by a certain number of degrees plus some losses to the environment). Those numbers will always be equal, no matter what. It is the same in every single field of science, every technology out there, every application of physics really. Energy in is always equal to energy out, no exceptions, ever.