This paper presents the methodology for economic system operation considering the cost of wear of transformers. It is another application of the economic use of equipment, a theory in power systems already published by one of the authors, though, using the wear of cables as an example. According to this theory, economic use guides system operation, so that the cost of equipment wear is considered during minimization along the usual operating costs. In order for this purpose to be achieved a mathematical description of the connection between wear and use is needed. Power transformers are another costly part of equipment that wear can be expressed as a function of its operation. Similarly to our previous work concerning wear cost of cables, transformer wear is a process triggered by the supply of heat. The widely used reaction rate equation of Arrhenius is adopted for the calculation of insulation degradation rate due to transformer loading. In essence, it is based on the calculation of the probability that a collision of molecules participating in the degradation chemical reaction will actually result to a reaction with respect to temperature. If a cost due to wear is attached to the loading of transformers; the utilization of transformers can become a part of economic system operation control. A simplistic example demonstrates how the proposed theory can be used for the minimization of the overall cost of wear of transformers and generation.