The proportional pressure control of centrifugal pumps may significantly increase the efficiency of water supply systems in terms of both electrical energy consumption and water leakage. The goal of this research is the derivation of the approximation concept for evaluation of the efficiency improvement potential applying the proportional pressure control for variable speed pumps in water supply. The proportional pressure control has been compared with the constant pressure control. For this reason, the energy calculation analyses have been made for certain pumps. The concept for evaluation of the efficiency improvement potential has been derived and experimentally tested. The interaction among the energy consumption, water leakage, and declination of the pump proportional pressure control curve has been found at the specific load profile. Based on the average possible linear declination of the pump proportional pressure control curve (15%) and the average water leakage in network (24%) in Latvia, the conclusions are as follows: 1) 12.2% of annually consumed electrical energy may be saved up in water supply systems applying the proportional pressure control is used instead of the constant pressure control; 2) The annual reduction of the existing water leakage rate may reach 3.7% in water supply networks.