Uninterruptible power is supplied to a load connected to a power line by connecting the power line to a rotatable member that includes a Rankine cycle turbine coupled to a device that operates as a motor when line power is operative thereby rotating the member at a standby rotational speed so that a predetermined amount of kinetic energy is stored in the rotating member, and that operates as a generator when the line power is inoperative and the member is rotated by the application of vaporized working fluid to the turbine. Working fluid is vaporized and maintained at an operational temperature level only when the line power is inoperative. The working fluid is held at a standby temperature level, preferably greater than the operational temperature level, while the power line is operative whereby the working fluid contains a predetermined amount of stored thermal energy while the power line is operative. In response to loss of line power, vaporized working fluid is applied to the turbine thereby rotating the rotatable member whereby the turbine rotates the member at a nominal operational speed that preferably is less than the standby rotational speed. Finally, in response to the loss of line power the device is connected to the load whereby the stored thermal energy in the working fluid and the stored kinetic energy in the rotating member are converted by the device to power for the load.