The efficiency of the fossil gas boiler is measured as the ratio between the useful output (heat), and the input (fossil gas). We used the following average efficiencies:  – Fossil gas boiler in the UK: 82.5% based on DECC (2014). – Fossil gas boiler in the US: 85% based on STRATEGO (2015). The conversion efficiency of a boiler or furnace cannot be more than 100%. The air conditioner, however, functions in a different way. Instead of burning fuel to create heat (like the boiler), it uses electricity to move heat from one place to another (indoor to outdoor, in the case with cooling). Technically, an air conditioner works as a heat pump, and can have an efficiency above 100%. We assumed that the US air conditioner has a conversion efficiency of 310% based on STRATEGO (2015). This means that for every unit of electricity supplied, the air conditioner transfers more than three times as much heat from inside to outside the house, thereby cooling the house.