The objective of this paper is to propose a new approach for defining the physical meaning of the basic electric power quality indices, determined by existing standards. According to the proposed approach the basic electric power quality indices represent the relative value of the distortion energy in a specific frequency interval, which affects different types of electric power equipment. Previously, changes in frequency characteristics of the voltage distortion and the electric equipment were taken into consideration only in very specific cases e.g. under estimation of flickers. However, these changes in frequency characteristics should be taken into consideration in all cases. The corresponding influence of voltage distortion has to be estimated during specific period of time; that is why it is necessary to analyze energy of distortion. Its allowed mean value corresponds to the norms of electric power quality indices.