The growing deployment of variable renewables has created both need and opportunity for affordable energy storage over multi-day and longer durations, which may then enable renewable power plants that are is cost-competitive with fossil fuel generation. To reach this goal, stationary storage is desired that adds a cost of ~$0.03/kWh-cycle or less to the cost of renewable generation. However, at long durations, the decreasing number of operating cycles over lifetime dictates that the capital cost of a storage system must proportionally decrease in order to meet such metrics. Storage technologies are desired that have exceptionally low cost of stored energy, while operating at much lower C-rates than for most battery applications. Flow batteries have the flexibility of design to meet these requirements if the underlying chemical cost of storage is low enough. This talk will give examples of use-case and techno-economic analyses that help to define requirements, and report on progress towards these goals using aqueous sulfur based flow chemistries.