A heat pump takes the ambient / outside air temperature and 'upgrades' or increases it, to heat air or water. The measurement of a heat pump's efficiency is its 'Coefficient Of Performance' or 'COP value'. The higher this number, the more efficient the system.
For example, consider a simple electric bar heater. All the electricity it consumes is converted to heat, therefore its COP value = 1. However, today's 'inverter' air conditioning systems have a typical COP value of around = 3.5. In effect this means that for every kilowatt of electricity they consume, they provide around 3.5kw of heat energy. They are therefore a highly efficient means of home heating.
In fact, 3 typical domestic air-con units only consume the same energy as a typical 2kW electric radiator – heating 3 rooms instead of one!
The same efficiency or COP value is also true for heat pumps used to heat water, for pools or even underfloor heating.