The problem under consideration is improvement of the energy efficiency of a control system of parallel-running pump units. Known methods used to solve this problem are considered. They can be divided into offline and online ones. As for the first group, the accurate model of the plant under consideration is needed. This is a non-trivial task under condition of the plant non-stationarity. In addition to this, such methods application is whether time-consuming or requires significant number of experiments with the real multipump unit. So online methods are becoming more and more popular, as they allow to take into consideration pump nonstationarity. But they are mostly used to choose the number of pumps to be turned on at the moment. The question of each pump frequency (speed) selection is still open. So, the idea of a new approach, which is based on the online optimization and allows to solve this problem, is proposed.