Supercapacitors are well suited for power backup applications, especially where battery replacement and maintenance cost is very high, or where very long life, and stable, predictable aging is needed. In power backup applications, energy requirements need to be low because of the low energy density. A good rule of thumb is for energy requirements of 60 seconds or less. An exception to this is where the backup power requirement is very low, such as in electronic applications. A good example of a power backup application is that used in Wind Turbines, for power in an emergency pitch. In this example, power is needed for about a half-minute when utility power is lost to the wind turbine, and the blades need to be turned to stop the rotor. Replacement cost is high, as the batteries are located up the tower in the hub. It is a two person task that takes a half-day to accomplish.