The bioenergetics efficiency of aerobic respiration of an organism is expressed through P/O ratio, which represents the ATP production per oxygen atom reduced in Electron Transport Chains (ETC). P/O ratio, to date, has not been extensively used in the context of the in silico analysis of metabolic networks, and most of the time it is considered as a fixed and imposed quantity. We studied the interplay between P/O ratio and the physiological states of the organism, using a consistently reduced E. coli core model with thermodynamic constraints which are applied to the model by utilizing an in-built toolbox called AGADOR. Plus, metabolite concentrations that govern the redox state of the cell as well as the media composition are integrated to the model. We determined the optimum in silico P/O ratio by imposing different values for this quantity; and we have found that the highest growth is achieved with a P/O ratio of 1.55, which is in accordance with experimental value as 1.5±0.1. In addition to this, maximum growth and optimum P/O correspond to the lowest heat dissipation for producing 1 Carbon mole of biomass. This result is also in accordance with the experiments that demonstrate the relation between carbon yield and corresponding dissipated heat. Our study demonstrates that P/O is not a fixed quantity, and it has a strong relation with the growth regime of the cell. The accordance of experimental and in silico results shows that the cell optimizes growth with optimum P/O ratio, but it also optimizes the heat disposal per 1 Carbon of biomass produced.