Abstract. Aim. The paper examines the methodological aspects of estimating the optimal reserve capacity in a concentrated electric power system. The sources of errors in the Markovich formula are identified. It is shown that the key requirement for the applicability of the Markovich formula is the assumption of absolute dependability of additional backup power units. The author analysed the mathematical procedures for identifying the probability and mathematical expectation (ME) of power shortage (PS). He proposed formulas that allow analytically identifying the MO and the PS variance. It is proposed to represent the schedule of scheduled repairs of main generation equipment as a deterministic component of the additional load, which allows abandoning the random processes model in favour of random variables. It is shown that there is an almost linear functional relationship between the probability and the ME of PS, which allows estimating the adequacy indicators based on the interval probability of the PS.