Pumped hydroelectric and compressed air energy storage (CAES) are currently economic for utilities when relying on natural geologic formations and the cheapest, most abundant substances (i.e. elevated water and compressed air). In these situations the cost of energy storage capacity can be very low (<$5/kWh1) Unfortunately the scale and location-specific nature of energy storage in natural formations is likely to render it of limited benefit to small scale distributed renewables. Available pumped hydro and CAES capacities could prove small in relation to the overall amount of future global renewable electricity?and attendant storage requirements?necessary for massive reductions in greenhouse gases. If energy storage in existing natural formations turns out to be insufficient, then man-made energy storage systems will be needed to support massive deployment of intermittent renewables.