Account of power losses arising by electricity supply concerns electricity producers and consumers. As a result of extensive studies, various methods to solve this problem are known. Here the method based on the increments of power losses is presented. This method requires knowledge of reciprocal current which flows from a supplier to a particular consumer. The increments of power losses are obtained increasing the node currents by a small increment of reciprocal currents. In such a way, the reciprocal losses, caused by power delivered by one supplier to a particular consumer are determined. In transmission lines, the reciprocal losses are determined in a similar way. Balance of currents observed, the sum of all reciprocal losses does not depend on reciprocal currents. However summary losses of a supplier or consumer can be determined by summation of its reciprocal losses but they depend on reciprocal currents. The choice of principle of reciprocal current definition is achieved by agreements between the parties.