An integrated sustainability criterion was developed to assess the sustainability of energy systems in a holistic manner. Many analytical methods of sustainability have been developed on the ecosystem-based values subject to the use of production factors, energy carriers, environmental services, and impacts. According to the basic definition of exergy - the maximum theoretical obtainable useful work - in the present research work, a new approach for assessing the exergetic performance of production factors was developed. The novelty of the developed Exergetic Opportunity Cost (EOC) approach was in defining the exergetic performance of the production factors as their capacity to substitute exergy destructions. The EOC was defined to be equivalent to the theoretically achievable reduction in exergy destructions resulted from gaining access to the production factors. The results were then compared with the Extended Exergy Accounting (EEA) developed in the literature. Both of the physical concepts (EEA and EOC) were applied in a case study in Iran to assess the exergetic cost of production factors in an energy system.