The problems of functioning of the IPS of Ukraine at fast growth in its structure the renewable energy source (RES)capacities such as wind and solar power plants are considered. In existing conditions, the presence of a significant share of basic nuclear generation, a small maneuverable range of TPP power units, and a limited amount of highly maneuverable hydro generation of the existing flexibility of the power system is not enough to ensure demand-supply balance of the power system with surplus RES electricity generation. The solution to this problem is currently performed by using hydro-pumped storage during uncharacteristic daytime hours of the day and limiting of electricity supply of wind and power plants into the power system. A modern solution is the use of to the battery storage systems to transfer the surplus of electricity from these RES in hours of increased consumption. The article presents the developed approach to modeling the participation of wind and solar capacities in load coverage balances of the optimization model of the forecast structure of power system generating capacities, that takes into account both curtailment and accumulation of excess generation of these RES. The modeling results showed that with significant of wind and solar power capacities, providing balance coverage load during spring floods with a significant share of nuclear generation capacity and the need for water supply is carried out as using hydro-pumped capacity during daytime hours of surplus capacity, grid battery storage systems so and battery storage systems, working together with RES, and their power curtailment. Keywords: power system, balance reliability, mathematical model, battery energy storage system, load curve, power curtailment