As the operation of efficiency power plant（EPP） is random， therefore， the output of EPP cannot be calculated simply with the capacity difference between conventional equipment and energy-saving equipment. Rather， the output curve of an EPP is obtained by monitoring its operation. The analytical method is used to calculate the reliability of power systems. First the output curve monitored by the EPP is separated from the load curve. Then the power system reliability indices are calculated by the net load curve after the separation and outage table of the conventional power plant. Meanwhile， by use of the relationship between loss of load expectation and annual peak load， the effective capacity of EPP in the power generation system is calculated. The way the above-mentioned idea is illustrated with an example.