The application of analytical procedures, those are based on the probability theory definitions and theorems, is considered in the framework of the bulk power systems adequacy assessment problem. The proportional load shedding strategy is accepted as the main one for obtaining a power flow distribution. In addition, the distribution of power flows is proportional to the difference in the angles of the voltage vectors at the nodes of the electrical network. The article considers the problem of accounting power losses in the process of power systems adequacy assessment. It is shown that the power losses accounting significantly affects the resultant adequacy indices of bulk power systems and therefore calculating of power losses has to be an inalienable part of the adequacy assessment process. The proposed mathematical models and algorithms are tested. For comparison purposes, Monte Carlo simulation technique is used as a bench-mark. The comparison of test results shows the high efficiency and acceptable accuracy of the proposed combined probabilistic analytical method.