This report derives thermodynamic relations that give information on questions of interest like the sustainability of the energy system, the accounting of their external costs or the harmonization between renewable energies and fossil fuels. Three important consequences of this work are, on the one hand, the existence of objective conditioners that help to the preselection of fuel candidates to be burned or solar synthesized in a certain location; for sustainability, the best fuel to be burned is carbon, whereas the one to be solar synthesized is hydrogen. On the other hand, the processing of the cycle of fuels replaced by solar synthesis, which takes place without chemical or mechanical environmental impacts, whereas the net environmental thermal impact (avoided environmental heating) is equal to the produced net work (null in the worse of the cases). Finally, the fuel replacement by solar synthesis allows to establish objective criteria of valuation of the external costs of the fossil fuel combustion. The integrating consideration between fuels suitable for their solar synthesis or for their combustion (each one of them depending on the climatic characteristics of the location, reason why they do not have to be the same) suggests the use of the fuel more suitable for solar synthesis as a chemical contribution for the replacement of the most advisable fuel for its final consumption; this consideration supposes the establishment of a parallelism between the previous results, consequence exclusively of Thermodynamics, and the corresponding ones to the biological photosynthesis, according to Biochemistry. (C) 2003 Elsevier Ltd. All rights reserved.