An Air Source Heat Pump (ASHP) will typically produce around 4kW thermal energy for every 1kW of electrical energy consumed, giving an effective “efficiency” of 400%. It is thermodynamically impossible to have an efficiency of more than 100%, as this implies that more energy is being produced than is being put in. For this reason the performance is expressed as a Coefficient of Performance (COP) rather than an efficiency. The above example would be expressed as having a COP of 4. The reason that it appears that more energy is being produced than is consumed, is because the only “valuable” energy input is electricity used to drive the compressor and circulating pumps. The remainder of the energy simply transferred from a heat source that would otherwise not be used (such as the ambient air, ground or a river) so is not considered as an energy input.