Recent attempts to construct national energy efficiency indices begin with the construction of "Btu aggregates" which are developed by adding together different energy resources based on their heating potential values or the heating values of the primary energy resources used to produce the energy resources which are ultimately consumed. The resulting indices may be of limited use in economic studies, where it is often important to consider the relative economic value of various component resources and their substitutability in response to relative price changes. In such applications, Btu aggregates will tend to suggest greater achievements in energy efficiency during periods of electrification than would an approach which aggregates different energy resources based on their market values.