These systems can be highly efficient delivering up to 250% seasonal efficiency. However, as the external temperature falls the operating efficiency falls too.

The performance of an ASHP is often quoted with "A7" (this being the air at 7°C). Performance should be based on EN14511, which will demonstrate their true performance. Furthermore, ASHP's are often quoted as "operating down to -15 deg C ambient". The output drops markedly at low external temperatures to the point where there is no effective heat at all since the heat pump is almost permanently in defrost mode. As a result, the efficiency declines significantly.

If an ASHP is replacing an existing system, the carbon factor of the fuel must be considered. Converting from a conventional gas system could actually be worse since the change from gas to electricity as a primary fuel source means an increase of CO₂ concentration per kWh based on UK Building Regulations 2016 data. Gas consumption emits 0.194kg of CO₂ for every kWh used, whereas electricity generation emits 0.43kg per kWh, more than 200% increase! The system improvements have to counter this before any savings can be made.