The Energy Mix Concentration Index (EMCI) is a quantitative indicator of concentration of the energy mix based upon the Herfindahl-Hirschman Index. We use the EMCI to compare the evolution of the diversification (versus concentration) of energy mixes in the long-term in order to reveal the transformations of the energy structures which determine energy transitions. In this methodological paper we make explicit how to aggregate the energy sources in order to calculate the EMCI, including questions of detail such us the level of aggregation and the transformation of primary electricity to add it up to total consumption. We present alternative figures that illustrate some additional aspects of the relation of the EMCI to total consumption, consumption per capita and energy annual growth. We also show the sensitivity of the indicator to alternative specifications (with and without pre-modern energy sources) and alternative data sets, proving its robustness. •Indicate how to aggregate energy carriers in the calculation of a quantitative index of concentration of the energy mix.•Compare alternative specifications (with or without pre-modern energy carriers).•EMCI focus on the major energy sources in the energy systems.
Keywords: Decarbonization; Energy concentration; Energy consumption; Energy diversification; Energy history; Energy mix; Energy security; Energy transition; Herfindahl-Hirschman Index.