A methodology that gives the thermodynamic limits to the amount of solar radiation needed to synthesize a pure fuel is proposed. This methodology is based on the concept of "solar equivalent of a fuel", and is related with the reversible reaction of synthesis of the fuel from environmental reactants and solar radiation. Solar equivalent of a fuel is defined as the minimum amount of solar energy needed to synthesize one mole of the pure fuel, at environmental temperature and pressure, from reactants and with the other products contained in the environment, when this one and the Sun are the only energy sources. Through out this paper, the environment is considered as a real mixture of gases, although the methodology can be easily extended to solid or liquid mixtures, or a combination of the different aggregation stares. Thermodynamic algorithms depends only on Sun radiation equivalent temperature, thermal equations of state of environment and pure fuel, and standard equilibrium constant of formation for each compound taking part in the chemical reaction. Numerical and graphical results are given for the synthesis of graphite, carbon monoxide, methane, ethane, propane, methanol, ethanol, and dihydrogen, all of them synthesized from atmospheric carbon dioxide and water vapour. The results help to consider under which conditions a fossil fuel should be used when primary energy can be directly solar. Those results give an objective criterium to valorize economically the replenishment costs of fossil fuel in relation to economical costs of solar technologies.