A battery-powered device that sends power to a computer system when the normal power source (usually the utility main) fails. As an auxiliary power supply, a UPS provides instant or near-instant continuation of electrical current after a power failure. There are two distinct types of UPS, off-line and line-interactive (also called on-line). An off-line UPS remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. An on-line 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. The on-line type of UPS, in addition to providing protection against complete failure of the utility supply, provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner.