Transformer efficiency, which has an impact also on the environment, involves minimum of transformer power losses. The losses are main factor also in transformer owning cost. They are minimal when ratio κ of load loss to no-load loss assumes a certain value. This value depends on how transformer load changes when operating a transformer. However, for minimum of transformer losses and its owning cost minimum, the ratio κ is different. The owning cost (total) of a transformer is determined more accurately; for this, to optimization is brought not only transformer loss price but the price of transformer active materials as well.