Nowadays, the overhead expenses are a significant component of the manufacturing cost. The expenditures occasioned by running the shop floor energy system are one of the most important among the overhead expenses. However, the manufacturing process optimization actually does not take into account these expenses. More than that, the expenses for running the shop floor energy system are not constant, varying after the external environment seasonal conditions, and they are depending on several issues, like the building's energetic exchange with its environment, or the loading degree of the machine tools from the shop floor. This paper approaches the relation between the shop floor energy system expenditures, on one hand, and the manufacturing process optimality, on the other hand. A case study, revealing the influence of the shop floor energy system on the manufacturing cost-optimal solution is also included.