The method of analysis of gases dissolved in oil is one of the most informative methods for early detection of defects in power oil-filled transformers. This determined its wide application in the practice of off-line and on-line monitoring of transformer equipment. In the article, based on the ratio of weight coefficients used in the calculation of the technical condition index of functional units and equipment units, a method for determining the reliability of oil-filled transformer equipment is proposed. An indicator that characterizes the quantitative measure of the diagnostic value of a single method (group of methods) for monitoring parameters of a power oil-filled transformer in the calculations of the technical condition index is obtained. The efficiency of monitoring the state of a power oil-filled transformer was evaluated based on the results of the analysis of gases dissolved in oil. A quantitative example of calculating the economic effect of increasing the operational reliability of a group of 110 kV power transformers in one of the areas of the distribution network using a remote diagnostic monitoring system based on the analysis of gases dissolved in oil is given.