Abstract The substantial increase in power generation from variable renewable sources has led to renewed interest in energy storage. Pumped hydropower remains the only mature and widely-adopted utility-scale energy storage technology. However, the selection and development of new pumped hydropower sites is heavily influenced by physical constraints such as terrain, as well as non-physical considerations such as the proximity of proposed reservoirs to settlements or environmentally or culturally sensitive sites. Hence, transforming existing reservoirs to pumped hydropower schemes is often considerably easier than developing completely new schemes. A model is proposed to calculate theoretical potential of a large area for the development of pumped hydropower schemes from existing conventional hydropower stations and from non-hydropower reservoirs. The methodology combines a new database of existing dams and reservoirs with a digital terrain model to identify suitable reservoirs for transformation, applies several constraints to eliminate unfeasible sites, then calculates the realisable potential in terms of power output and energy storage. The model is tested by applying it to the case of Turkey to produce country-level estimates of the theoretical and realisable potential for such transformations. In excess of 3800 GWh of realisable energy storage potential was identified from over 400 sites in the country.