A modular flow battery includes a battery stack container housing a plurality of redox flow battery stacks in fluid communication with at least one pair of electrolyte containers including an anolyte container for holding an anolyte and a catholyte container for holding a catholyte. Additional pairs of electrolyte containers can be connected to the battery stack container to increase an amount of energy that can be stored by the modular flow battery system. Respective housings enclosing each of the battery stack container and the electrolyte containers are configured for operation in a stacked configuration. In this manner, the energy storage capacity of the modular flow battery system can be further increased with substantially no increase in a lateral area occupied by the system.