The study considers the construction of a battery system within the existing wind farm connected to the transmission grid, which will enable the provision of a balancing service to HOPS in the form of offering aFRR power reserve in the amount of a minimum of 1 MW with the activation of balancing energy at their request. Through the developed optimization model, it is necessary to find the optimal combination of electricity sales on the day-ahead market and the regulated balancing energy market. Also, with regard to the estimated overall costs of the battery system and the optimal combined performance on the balancing energy market and the day-ahead market, it is necessary to find the optimal size of the battery that needs to be installed with the mentioned wind farm, which will ensure the fastest return on investment.