As the electricity demand constantly grows, the world is increasingly focusing on generating electrical energy from renewable sources, especially from wind and sun (fluctuating characteristic), whose share is continuously increasing. Additionally, market liberalization and various global crises can cause electricity prices fluctuations. One of the solutions to achieve a more secure and reliable power system due to the presence of a high share of fluctuating energy and the improvement due to electricity price volatility, is the energy storage use. In addition, it is necessary to develop efficient methods for optimizing the energy storage operation in order to achieve the desired goal. In this doctoral dissertation, a optimization model for energy storage operation in a market-oriented power system, is developed. The model does not focus on the specifics of individual technology, but optimizes the storage operation primarily for the daily charging and discharging cycle. The model was developed using the optimization technique of mixed-integer linear programming. The development of the general model aims to enable the user can make various simulations and optimizations of the energy storage in a market conditions, such as the optimization of the storage system only, the energy storage with some additional production sources and the overall system with all production sources. The model takes into account the operation of other commonly used production plants by modeling the operation of a single representative production plant that approximates of all other plants of the same type. The applicability of the developed model is presented through four groups of calculations. In the first group of calculations, the influence of one storage parameter on the storage optimal operating mode, was investigated. In the second group, it was shown what influence the energy storage has on the reduction of costs, due to the undelivered energy of production companies. The third group have analysed the impact of energy storage size on the operation of the overall system. The fourth group of the calculations have investigated the influence of the power system structure on the energy storage operation, specifically the increase in the share of storage hydropower plants production and the share of intermittent renewable energy production.