For the building interventions, a reduction in cost and carbon emissions is most directly associated with the resulting kWh reduction in heating fuel and/or electricity use. In this case, the calculation of carbon emissions is achieved through the use of carbon intensities (tCO2 equivalent per kWh) for either the direct combustion of fossil fuels on site, the purchased fuels and their distribution.
The DECC/HM Treasury Green Book guidance (as updated 11 December 2015) on valuing energy use and greenhouse emissions sets out projected costs and carbon intensities for gas and electricity (amongst other fuels) through to 2100 that have been used in the MACC calculations. Further detail on the assumptions behind the projects are detailed in the Background Documentation.
In summary, electricity costs are expected to increase over the timescales considered for the MACC calculations, whilst carbon intensities are expected to reduce. These costs and carbon intensities have been used where these are the only factors that have been used to calculate reductions in cost and carbon emissions, although they may arguably also be relevant for interventions where other factors influence costs and savings (for example, bed days, medicines waste etc).