Energy is conserved (under normal conditions) so everything is 100% efficient if you want to include waste heat. However, "efficiency" is actually a value judg

Energy is conserved (under normal conditions) so everything is 100% efficient if you want to include waste heat. However, "efficiency" is actually a value judgement. You probably want to heat something specific, so you're going to lose some fractional efficiency by accidentally heating something else. Resistance in the wires leading to your heater will accidentally heat the outside of your house. It's also not a terribly useful definition of efficiency environmentally: if you multiply the efficiency of burning fuel to produce the electricity by the efficiency of your electric heater, you get a worse result than burning fuel in your home. And it's not a terribly useful definition as a consumer either, since that electrical heater is only producing about 50% the heat that a heat pump could do with that electricity.