Following a Whole Building Approach to Decarbonisation will likely result in a fabric first approach in combination with the core heat decarbonisation project(s) (for example a heat pump(s) to replace fossil fuels) and some electricity saving projects (e.g. LED lighting, Solar PV).
For some of these energy conservation measures a whole building approach would be preferred (Option C) as it would be difficult to meter the impact of say a fabric improvement, whereas for electrical savings it is often possible to meter the equipment individually so a retrofit isolation approach would be more suitable (Option A or B). Heat pump(s) may have a whole facility (Option C) approach for heat savings but a retrofit isolation (Option B) approach for the input electricity and heat generated. The latter would also allow the system to be monitored and optimised in real time.
There is a further whole facility approach which is used where there is no baseline period: Option D - Calibrated Simulation. This involves using building energy software to predict facility energy use in the baseline period. This might typically be used to determine building performance relative to a standard for new buildings.
Apart from Option D, energy metering data should cover the baseline period and post-implementation (reporting) period. In addition, all routinely varying factors which drive consumption (e.g. heating degree days, which reflect the external air temperature) should be recorded for both periods, as well as any factors that could significantly influence the energy consumption but which do not routinely vary, such as building floor area, occupancy, etc. More comprehensive guidance on metering strategies is provided in CIBSE TM39 - Building Energy Metering (2009).