An energy storage system comprises a cable (13) and a mass (20) suspended from the cable (13) in a shaft (12). The cable (13) is attached to a winch (11) by which the mass may be raised in the shaft (12) to store potential energy, and the mass is lowerable in the shaft (12) to release the potential energy. The mass comprises at least two sections clamped together around the cable (13). More particularly, the system comprising a plurality of cable (13) and a plurality of multi-section masses each suspended in the shaft (12) by a respective cable (13), the masses being raised and lowered in synchronism in the shaft (12). The multi-section masses fit together side-by-side in the shaft (12) to form an overall cylindrical mass body, the multi-section masses each forming a quadrant of the cylindrical mass body.
