A modular energy storage system has: a battery module with a battery and internal circuitry; a control module with a power outlet, internal charge-and-discharge electrical components, and a power inlet for connection to a power source in use; the battery module defining a top seat that has an associated electrical connector; and the battery module being mounted to the control module below the control module by the top seat, whose respective associated electrical connector connects to the internal charge-and-discharge electrical components to permit the control module to: charge the battery module with power from the power source; and discharge the battery module by transferring power from the battery module to the power outlet.