PC Hardware in a Nutshell, Second Edition

Chapter 27. Backup Power Supplies

There really is a difference between an uninterruptable power supply (UPS) and a standby power supply (SPS), but common usage now designates a unit properly termed an SPS as a UPS. We call a unit of either sort a backup power supply (BPS), which neatly sidesteps the terminology problem.

A BPS comprises a battery and some supporting circuitry, and is designed to supply power to your PC for a short period if the utility power fails. This temporary reprieve allows you to save your work and shut down the PC in an orderly fashion. BPSes differ in the quality of the power they supply, how much power they can supply, and for how long they can supply it. BPSes also condition the utility power to protect equipment against spikes, surges, drops, brownouts, and electrical noise.

What BPSes Protect Against

Most electric utilities supply consistent, well-regulated power. But as that power moves from the generating plant through the distribution grid to you, the power company gradually loses control of its quality. A good BPS protects against all of the following power problems:

Blackout

A blackout is a sudden, complete loss of voltage, which may be accidental (a tree falling on a power line) or intentional (the power company shedding load during a power emergency). Blackouts are the reason most people consider buying a BPS, but they are the least common power problem. Blackouts of very short duration, called drops, occur frequently and often pass unnoticed. ...