Energy storage is increasingly viewed as a valuable asset for electricity grids composed of high fractions of intermittent sources, like wind turbines, or unreliable transmission and distribution services. However, the likelihood that batteries will meet the stringent cost and durability requirements of grid applications is largely unquantified. In this work, we investigate conceptual and actual aqueous and nonaqueous flow batteries designed to store energy for hours and analyze the relationships among technological performance characteristics, component cost factors, and system price. The benefit of high voltage associated with the nonaqueous approach must be balanced against the burden of high solubility of active materials. Requirements in harmony with economically effective energy storage are derived for both systems to facilitate comparison.