This paper addresses the electric power system, where selfish generation companies produce electricity and a system operator defines nodal prices of electricity and transmits electricity through the network to consumers. To optimize electricity prices and the amount of produced electricity, we propose a hierarchical (bilevel) model with the system operator at the upper level and the generation companies at the lower level. Every generation company wants to maximize its profit function depending on the price of electricity. The goal of the system operator is to define optimal nodal prices and network flows such that the generated electricity meets the demand at every node and at every period. In order to construct numerical methods for the problem in question, first, we transform the original bilevel problem into a single-level nonconvex optimization problem by replacing the lower level with its optimality conditions. Then we apply the Exact Penalization Theory and Global Search Theory (GST) to the resulting problem. Next, according to the GST, we construct the DC representations (the differences of two convex functions) of all nonconvex functions from the problem statement and develop local and global search methods taking into account the specific of the problem in question. Computational testing of the developed methods on a test problem demonstrated efficacy of the approach proposed.