An electrical heater is not a system, it's just part of one. The system itself would at least involve the energy source and the transporting medium, where the losses continue to hold true the fact that there's no 100% efficiency in any system. If there were, Entropy would have no meaning.
If you take the isolated case of the heater as being 100% efficient because in itself it's able to convert 100% of its input energy into wanted heat (which itself is discutable, since there may still be losses through electro-magnetic emissions, vibrations, etc. that would not be captured by the room and transformed into heat, even though all that would be as negligible as contesting gravity acceleration as not being 9.8m/s^2 in most cases), you'd just as well pick a heat pump which, with the same amount of input energy, "produces" 3 times more heat.