Device for storing energy, using a physical object, such as a mass or buoyant object floating in fluid. A mass is repositioned to greater altitude in a gravitational field to a position of higher potential energy. A buoyant object is forcibly submerged into a fluid, displacing fluid, to a position of higher potential energy. The stored potential energy may be recovered with extremely low loss regardless of the state of charge of the system, or length of time of the storage. Maintaining the charge is indefinitely lossless.