Energy storage system

The system (1) has a control unit (13) for controlling an intake (En) of energy from or output (Ep) of the energy to a non-local power grid (5) and local power grids (61, 62) according to stationary and non-stationary control and system tasks (LRS, NLRS). The control unit controls the non-stationary control and system tasks only within a framework of free non-local system storage capacities and a free non-local system output of total system storage capacities and/or a total system output that are not required for the stationary control and system tasks. An independent claim is also included for a method for operating an energy storage system.