This article illustrates an innovative approach for the characterization and comparison of the performance of power-supply systems. The concept of 'grammar' forces to declare the pre-analytical decisions about: (i) semantic and formal categories used for the accounting - primary energy sources (PES), energy carriers (EC), and production factors; (ii) the set of functional and structural elements of the power-supply system included in the analysis. After having tamed the systemic ambiguity associated with energy accounting, it becomes possible to generate a double assessment referring to: (i) external constraints - the consumption of PES and the generation of waste and pollution; and (ii) internal constraints - the requirements of production factors such as human labor, power capacity, internal consumption of EC for making EC. The case study provided compares the production of EC (electricity) with "nuclear energy" and "fossil energy". When considering internal constraints, nuclear energy requires about twice as much power capacity (5.9-9.5 kW/GWh vs. 2.6-2.9 kW/GWh) and 5-8 times more labor (570-640 h/GWh vs. 80-115 h/GWh). Things do not improve for nuclear energy when looking at external constraints - e.g. the relative scarcity of PES. This may explain the difficulties faced by nuclear energy to gain interest from investors. (C) 2012 Elsevier Ltd. All rights reserved.