||In the field of complex energetics, human societies to survive follow the same ‘maximum power principle’ as other living systems. In this view, human societies developed because they have been able to increase ‘their capacity to convert energy at a given time rate’ rather than simply increase ‘their level of energy consumption’. This was translated into an increase of the level of power capacity in human societies so far. Yet, one can expect that the level of power capacity will be altered in light of the unavoidable progressive depletion of fossil energy resources. The systemic study of power capacity in sustainability assessment is therefore essential for facing the external constraints ahead. This paper seeks to clarify the concept of power capacity in sustainability assessment. It provides explicit methods of assessment for the different types of power capacity used by human societies. Power capacity refers to the converters transforming energy flows at a given time rate. Dealing with societal transitions therefore requires being able to characterize properly those converters in addition to the study of energy flows. However, this requires extending the timescale typically considered in conventional energy analysis which entails several epistemological problems over sustainability assessment.