The problem of determining the probabilistic indices of power shortage in a concentrated electric power system (EPS) on the calculated time interval is considered. The difference between the concepts of 'point' (PPPS) and 'interval' (IPPS) probability of PS is shown. The multiplicity of definitions of the IPPS is presented. It is recommended to take the fraction of time at which the EPS is in a deficient state as an IPPS. Mathematical mechanisms for determining the interval probability of power shortage are analyzed. It is proposed to take the weighted average duration of the power shortage with the weights of the PPPS as the IPPS. A strong correlation is shown between the point probabilities of PS of the most dangerous regimes (maximum daily, weekly monthly or annual loads). © 2022 IEEE.