Due to their properties, the most suitable application for flow batteries currently is a bulk energy storage. This paper investigates the economic feasibility of the technology in terms of monetary profitability in the appropriate business cases, namely employment in energy markets and in isolated island systems with the high share of renewable generation. We calculate the flow batteries life cycle costs and compare them with the potential revenues from participation in the Finnish energy markets and operation in isolated power systems of the Faroe Islands and the island of Graciosa. We find that the flow batteries exploitation in the Finnish market is not profitable - they collect 43-60% of their costs in the most promising application. The island cases represent a more viable option due to the high fuel costs of the thermal plants that the batteries and renewable sources substitute or decrease their share. However, the revenue and subsequent profitability highly depend on the volatile fuel prices.