Heat pumps use the principle of thermodynamics to transfer heat from one place to another. In winter they extract heat energy from the outside air and transfer it inside. The same happens in the summer, but in reverse: they extract heat from inside and transfer it outside. Heat pumps are therefore more efficient than traditional electric heating systems that use electrical resistance to generate heat. A high-performance heat pump system produces about four times more heat than a resistive system for the same energy consumption, as well as offering the comfort of air conditioning during hotter weather.