Methods of probabilistic theory and mathematical statistics allow creating analytical models of graphs of electric loads, models of current and voltage change in power networks. These models are required for calculation of a number of integral characteristics of power networks operation modes, such as voltage measurement range, power consumption and losses and etc. Moreover, they allow solving problems of optimization of power systems operation modes, their development and power consumption planning. The paper considers a possibility of using a probabilistic and statistical method for calculation of power losses. As the existing methods are designed for calculation of single and three phase power networks with symmetric loads, the authors propose the probabilistic and statistical method of determining power losses for three phase distribution networks of non-tractive consumers that considers asymmetric nature of loads. The authors carried out an assessment of accuracy of the proposed approach and determined the sphere of its application.