The definition of the residual life of electrical equipment is given as the total operating time from the moment of monitoring its technical condition to the transition to the limit state. The article highlights the importance of developing methods for calculating this parameter for power transformers. The relevance of solving this problem is becoming higher by the introduction of on-line monitoring systems for technical condition. They expand the possibilities of predicting the equipment life during operation based on a set of diagnostic parameters. The article provides the definition for the equipment resistance to failure which consists in maintaining its operability within a specified period. Analytical dependencies for calculating this indicator and a graph of its change in case of defects are given. Expressions are obtained for calculating the operating time for failure and the remaining life. An example of calculating these parameters for an 80 MVA power transformer is given. The initial data includes DGA indicators for 257 days of continuous operation. They are recorded by the on-line condition monitoring system implemented on this unit. Finally, the authors draw a conclusion on a satisfactory performance of the transformer within the controlled period. A dimensionless indicator is proposed: a dynamic coefficient of relative resistance, which determines the intensity of the decrease in the resistance to failure. Dependencies are given for calculating it when one or more defects occur. The curves of changes in the relative resistance to failure for the transformer under study are provided. As a result, the conclusion about its normal technical condition was confirmed. The application of the developed methodology combined with on-line condition monitoring makes it possible to forecast the transformer's life during operation with a given frequency. This is a scientific and practical contribution of the research conducted.