PC Hardware in a Nutshell, Second Edition

Choosing a BPS

Use the following guidelines when choosing a BPS:

Select BPS type according to application

On-line and line-interactive units are too large and too expensive for most PC applications. Consider them only for enterprise/departmental servers and other critical systems. For standard PCs and workgroup servers, buy an off-line unit. If your location is subject to frequent power problems and you can afford to do so, choose a line-boost unit, which greatly extends run time under brownout conditions.

Pick a unit with adequate VA and run time

You can calculate VA requirements by checking the maximum amperage listed on the PC power supply and on each other component the UPS will power. Total these maximum amperages and multiply by the nominal AC voltage to determine VA requirements. The problems with this method are that it is time-consuming and results in a much higher VA than you actually need. For example, a typical 250W PC power supply that actually draws about 375 VA (250/0.67) when fully loaded may list 8A maximum draw, which translates to nearly 1000VA. A better method is to use one of the sizing tools that most BPS makers provide on their web sites. For example, the APC UPS Selector (http://www.apc.com/sizing/selectors.cfm) allows you to specify your system configuration, the run time you need, and an allowance for growth. From that information, it returns a list of suitable APC models, with the estimated run times for each. Or, for a quick-and-dirty selection, simply ...