UPS: Uninterruptible power supply

In short: A UPS bridges power outages with a battery so that computers and servers can shut down cleanly instead of going out abruptly.

In more detail: A sudden power failure can damage files, destroy ongoing write operations and stress hardware. The UPS sits between the socket and the device, keeps the voltage constant and, in case of failure, supplies power from its battery, usually for a few minutes.

In Depth

Types

• Offline / standby: the device normally hangs on the mains; on failure the UPS switches to battery within a few milliseconds. Cheap, for home PCs and routers.
• Line-interactive: like offline but also automatically smooths out small voltage fluctuations (AVR). A good middle ground.
• Online (double conversion): the power always flows through battery and inverter, so the device always sees clean voltage, without switchover time. Expensive, for servers and critical systems.

Choosing the right size

The capacity is stated in VA (volt-amperes) and watts; the watt figure is decisive. Add up the consumption of the connected devices (rating plate or measuring device) with a reserve (about 20 to 30 %). The runtime depends on the battery; at full load often only a few minutes, at half load considerably longer. The aim is a clean shutdown, not hours of operation.

Interplay with the computer

Through a USB or network connection the UPS tells the computer about the power failure. Software such as NUT (Network UPS Tools, Linux) or the Windows power settings shuts the system down automatically at low battery. Servers and NAS can be set up centrally this way.

Care and notes

Batteries (usually lead-gel) last three to five years and must be replaced, otherwise the UPS will not hold in an emergency. Trigger the self-test regularly. Not on the UPS: laser printers, fan heaters and other heavy consumers. Surge protection against lightning is something different: a simple power strip with protection is not always enough. Only sensible for devices where a sudden stop does damage (servers, NAS, home computers with open documents).

See also: Power supply and 80 Plus, NAS server, Backups for beginners