The present disclosure is concerned with operation of an energy storage system (ESS) connectable to an electric power system. The charging-discharging schedule of the ESS can be determined through application of a time-dependent forecast of ESS and power system states based on historical data. Exemplary embodiments can include ESSs which have been historically activated with a certain periodicity, for example, for power system load leveling, frequency regulation, arbitrage, peak load shaving and/or integration of renewable power generation.