The consumption of primary energy is often used to compare energy systems from a sustainability viewpoint at regional, national or even world level. For such comparisons to be “fair” and meaningful, this implies however to define primary energies in a coherent way for the various possible sources (coal, oil, gas, nuclear, renewables). This paper stems from the achnowledgement that the definitions presently used in the framework of energy policy studies do not really fulfill this requirement, and suggests approaches to correct such a methodological weakness; the importance of considering exergy as a key concept in this context is particularly emphasized. The authors’ reflections in this matter originated from the recent adoption by the Board of the Swiss Federal Institutes of Technology, soon followed by the Swiss Advisory Committee for Energy Research, of the objective to achieve a “2 kW- society” (2 kW-year/year) in Switzerland by 2050. Such a target, which is about 2.5 times smaller than today’s consumption, invites to better define what such a number concretely means. The concept of useful energy is also briefly