An Uninterruptible Power Supply (UPS), also known as an Uninterruptible Power Source, Continuous Power Supply (CPS) or a UPS battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. UPS Battery Backups typically remain idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. Also, some UPS continuously powers the protected load from its reserves (usually lead-acid batteries or stored kinetic energy), while simultaneously replenishing the reserves from the AC power.