Modular energy storage system

An energy storage system includes modular energy storage equipment that may be connected to an external system, such as a power grid. In at least one embodiment, the energy storage system includes a power transfer control system comprising a power transfer network and a processing module or controller. The power transfer network has a first interface coupleable to one or more energy storage units and a second interface coupleable to one or more power conversion units. One or more conductors in the power transfer network are selectively coupleable to the first interface and the second interface for transferring power between the energy storage units and the power conversion units based at least in part on information indicating a power or energy supply or demand of an external system or information indicating an amount of energy stored in the energy storage units. The energy storage system is scalable for different implementations.