Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance; full-cycle lifetimes quoted for flywheels range from in excess of 10, up to 10, cycles of use), high (100–130 W·h/kg, or 360–500 kJ/kg), and large maximum power output. The (ratio of energy out per energy in) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 to 13.
