Environmental concerns have rekindled interest in energy conservation. The central question is efficiency, and calculation of efficiency requires a numerator: the absolute minimum primary fuel necessary to produce a given product or to provide a given service. Most articles on energy conservation address this question through classical thermodynamics, with Gibbs's notion of Availability (Keenan, 1951,1941, 1932).1 Classical thermodynamics treats systems in stable equilibrium, whereas the physical operations of production are designed to place systems in extreme nonequilibrium states. The classical approach is limited and can obscure possible modes of technical change exploiting nonequilibrium phenomena which can greatly increase efficiency. Immediate adoption of particular examples of energy conservation that classical analyses now reveals, as some advocate, would in some cases obviate use of the same capital to exploit richer opportunities for gains in both energy efficiency and general economic efficiency on the verge of being discovered.