A UPS (uninterruptible power supply) protects the loads connected to it from power outages. The UPS has a battery pack that is charged when the mains power supply is present. When AC power fluctuates or fails, the UPS automatically powers the connected load using energy stored in the battery pack. When mains power returns, the battery is recharged. The amount of backup time (runtime) available from the UPS is measured in minutes and is dependnent upon the size of the battery pack, measured in Ampere-hours (Ah) and the load size, measued in VA (apparent power) or Watts (real power).