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.

The most common 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. Most UPS for the home and small business systems are below the 700VA size, and for business and network servers a unit with more than 700VA would be recommended. Manufacturers also make rack mount UPS and power distribution units.

While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts). Larger UPS units typically work in conjunction with generators.