A device placed in series with the supply of power from the utility which contains energy storage such that the supply of power from the UPS is continuous even when the utility supply is removed. While battery-based energy storage is the most common, flywheel-based energy storage is gaining in popularity due to the reduced maintenance cost. UPS is used to hold the load until either the generator turns on to handle the load or the utility power is restored. Battery based UPS systems have a finite time that they can handle the load, usually less than 10 minutes. Flywheel-based energy storage is usually less than 5 minutes depending on the design.