Acronym for Uninterruptable Power Supply.

Roughly speaking, this is a big battery that will run your computer for a short time when the power goes out. It isn’t usually large enough to maintain power to the computer for an extended length of time (hours) (unless your money:IQ ratio is too high, or, like zcat, you've been given four perfectly OK 21-plate truck batteries, which he says should sustain 12 hours of standby power, even with the screen on). It is there to handle short power outages (<10 minutes) and then allow the machine to shut down cleanly.

There are two major types:

• Online UPS is constantly charging the battery from the mains and at the same time drawing power from the battery to run the computer(s).
• Offline UPS keeps a charged battery around to switch it inline when the power drops below a certain level.

Online UPS will therefore isolate your computer from any power spikes or brownouts, whereas offline UPS usually protects it against brownouts but hardly from spikes.

See also UPSNotes