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The evaluation and comparative analyses of energy conversion technologies are carried out with well-established concepts, like the concept of efficiency. Today, however, new technologies based on renewable energy sources challenge conventional approaches. Accounting for fuel diversity and other inequalities can ensure that comparative analyses result in trustworthy results. This paper aims to address this issue by updating conventional efficiency for more accurate comparative evaluations of fossil fuel energy systems. Specifically, the concept of total efficiency is introduced to account for extraction, processing and transportation of fossil fuels, aspects that are not currently considered in the conventional definition of efficiency. The total efficiency is determined by studying the environmental impacts of these processes and quantifying their energy requirements in terms of additional fossil fuel use. Two case studies in Spain are presented to demonstrate the impact of using the developed method on efficiency estimates. The total efficiency is found to be lower than the convectional efficiency by 21–25% to up to 41%, in the most unfavorable cases. This shows the significant impact of the preparation steps of fossil fuels and represents a fairer comparison between fossil fuels and renewable conversion processes. • Introducing total efficiency for more accurate comparative evaluations of energy systems. • Total efficiency accounts for extraction, processing, and transportation of fossil fuels. • The developed method links environmental impacts to additional fuel input. • Fuel processing decreases the efficiencies of fossil power plants significantly. • Updated tools, like total efficiency, are necessary to achieve future energy sustainability. [ABSTRACT FROM AUTHOR]