In 2003, the UK government announced its aspiration for a 60% reduction in CO2 emissions by 2050 relative to 1990 levels. To achieve this radical target, action is required across all sectors of the economy to reduce energy demand significantly and to increase the supply of energy from zero or low carbon sources. Focusing on the domestic sector, where energy consumption is currently rising, technologies such as fuel cells, Stirling and internal combustion engine micro-CHP and heat pumps are often cited as the means to reduce carbon emissions. However, there is much uncertainty as to the potential environmental benefits (if any) of the aforementioned technologies when set against a picture of changing energy supply and demand. The paper describes an analysis in which the performance of the four different technologies mentioned above was compared against a common datum of energy supply from condensing gas boilers and grid electricity for a number of scenarios. The aim of the analysis was to determine if significant CO2 savings could be made and to determine the minimum thermodynamic performance criteria that these technologies must attain if they are to yield any environmental benefits. The main finding of the work is that air source heat pumps yield significantly more CO2 savings than any of the other technologies examined.