Abstract: An energy or water storage system can comprise a storage reservoir, a discharge reservoir, a pump for charging the storage reservoir, and a turbine or similar discharge device. The storage reservoir can comprise a flexible bag covered by overburden that creates pressure inside the bag. Energy is stored by raising the overburden. The bag can comprise a shaped internal filler piece to prevent formation of a crease at the bag edge. Solar panels can be mounted above any bag to make additional use of the land. Solar panels mounted on overburden can have tracking mechanisms and controls suitable to achieve desired orientation regardless of the slope of overburden. Elevation difference between the storage reservoir and discharge reservoir can be used, including an underground cavity. The bag can have internal tethers to influence bag shape.