An uninterruptible power supply flywheel (120) includes a solid steel cylindrical flywheel (121) levitated for rotation about a vertical axis at tip speeds greater than 250 m/s on magnetic bearings (124, 125) in a low pressure housing (123). The flywheel inertia section operates with a maximum stress of greater than 60% and less than 80% of the material yield stress when spinning at fully charged operating speed. The flywheel is constructed from steel with an ultimate strength greater than 150 ksi and a toughness greater than 100ksi (in) &cir& 1/2. The flywheel is a solid steel cylinder that is hardened to centerline structure throughout the axial thickness of the large diameter portion that is greater than 40 % martensite. The flywheel is a solid steel cylinder that, in normal, fully charged operation, spins at a speed that would cause a fracture failure no sooner than one hundred thousand cycles between the fully charged normal operating speed and 10% of that normal operating speed. The flywheel is free of flaws with length greater than 1/8th inch at a diameter less than 20% of the outer diameter of the flywheel.