Non-sinusoidal voltage in electrical networks is one of the most pressing problems in the field of power quality. In non-sinusoidal modes in electrical networks, in addition to losses of active power at the fundamental frequency, i.e. at the 1-st harmonic, active power losses occur at frequencies that are multiples of the fundamental frequency, corresponding to harmonics from 2 to 40. Additional harmonic active power losses increase losses in electrical networks, creating economic damage for both energy companies and consumers. The paper provides a review of the literature on the topic under consideration. A methodological approach is proposed to assess additional active power losses in overhead power lines. Based on the methodological approach, an analysis of the power quality indicators characterizing the non-sinusoidal mode, measured at the point of connection of 110 kV transmission line to the electrical network, was carried out, additional active power losses were determined.