The rapid growth of renewable generation share in the power systems worldwide, that could be explained by the world community desire for sustainable clean energy, combating the global climate change, has led to widespread use of wind and solar power plants as today’s most productive and competitive. With significant volumes of generation of these RES there are difficulties in the power system balancing due to insufficient flexibility of traditional generation, which limits electricity supply from renewables. The electricity storage is seemed to be a key solution to a number of technical and economic problems of the wind and solar power integration into the power system due to its unique ability to store and re-supply electricity to the grid. Electricity storage can be used to store excess volumes of renewable energy generation, which will be used later, minimizing or removing restrictions on supply of that excesses. The article presents approaches to taking into account the use of storage systems of electricity storage in a partial-integer model for forecasting of the power system generating capacity structure development. The use of storage system for the consumption/supply of electricity in the daily electrical load curves of the power system is modeled taking into account the available storage capacity. For hydropower the limitation of the water resource availability is taken into account. Battery energy storages ensure the transfer of excess power from wind and solar plants to the peaking periods of daily load if there is absent the possibility to ramp-down conventional generation during maximum wind and solar electric power production. Keywords: power system, the long-term development forecasting, mathematical model, energy storage system, load curve