Building energy use to be offset or balanced is measured at its source of generation.

Source energy denotes the raw fuel used by the building to operate. In case of utility supplied energy, the source is at power plants and in case of captive energy generation, at the location of captive generation equipment.

It is computed by multiplying site energy with regional or national site to source conversion factors. Conversion multipliers are based primarily on source fuel type (coal, diesel, nuclear, hydro etc.). Transmission losses, delivery distance, production method and its efficiency are also factors that influence site to source ratios. Source energy quantification may be prone to errors unless conversion multipliers are derived through rigorous analysis.

Estimates for site to source energy ratios for major energy carriers like electricity are currently unavailable for India.  International conversion factors can be used for instead for calculations.