In this paper, a thermoeconomic optimization and a cost-benefit balance to sanction the substitution of fossil fuel-fed energy systems (PEES) by systems directly maintained by renewable sources (RSES) is developed. Inside the thermodynamic frame (eta, eta(c)) (W-max, 0), the multiparameter thermodynamic function phi = W-max(1 - mu/eta) determines the thermoeconomic optimization. The cost-benefit balance will sanction the substitution of PEES by RSES. To make this possible, the initial investment surplus, [NBf-NBr](t=0), in favour of RSES must be compensated in a reasonable period of lime t with the savings of fuel and environmental damages accumulated during that period, in favour of FFES.