A method and system for storing and/or discharging electrical energy that has a cost, which method includes steps of: (a) providing a flow battery system comprising at least one flow battery cell and a controller; (b) operating the flow battery cell at a power density having a first value; and (c) changing the power density at which the flow battery cell is operated from the first value to a second value as a function of the cost of the electrical energy, wherein the power density is changed using the controller, and wherein the second value is different than the first value.