Use the following guidelines when choosing a BPS:
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.
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 use the following guidelines:
For a low-profile, desktop, or mini-tower system with a slow processor, 128 MB of RAM, one IDE hard disk, zero or one expansion cards, and a 15- or 17-inch monitor, choose a 280VA to 420VA unit.
For a desktop or mini-tower system with a midrange processor, 256 MB of RAM, one or two IDE hard drives, one or two expansion cards, and a 17- or 19-inch monitor, choose a 350VA to 600VA unit.
For a mini-tower or tower system with one processor, 256 MB to 512 MB of RAM, one or two fast ATA or SCSI hard drives, several expansion cards, and a 19-inch or larger monitor, choose a 500VA to 700VA unit.
For a mini-tower or tower system with dual processors, 512 MB or more of RAM, two or three fast ATA or SCSI hard drives, several expansion cards, and a 19- to 21-inch monitor, choose a 650VA to 1000VA unit.
In each case, the smaller unit provides little reserve capacity for expansion, and may provide as little as five minutes of run time. The larger unit typically provides 30% to 50% reserve capacity for expansion, and run times of 15 to 25 minutes.
If you need to protect multiple PCs in close proximity, consider buying one or a few larger units rather than many inexpensive smaller units. The larger unit will probably cost less for the same cumulative VA and run time, and will likely provide superior features (e.g., line-boost and a better waveform). The only drawback to one large unit versus multiple smaller ones is that the larger unit will be able to shut down only one connected system automatically unless you also buy shutdown-sharing hardware, which is quite expensive.
The very cheapest units provide square wave output, which PC power supplies can use for short periods without damage. However, running a computer on square wave power for extended periods stresses the power supply and may eventually damage it. Also, square wave units are entirely unsuitable for other electronic devices, which they can quickly damage. Buy a square wave unit only if the alternative is not being able to afford a BPS at all. For general use, buy a unit that provides simulated sine wave if you expect to run the PC for 10 minutes or less on backup power before shutting it down. Buy a true sine wave unit if you expect to run the PC for extended periods on backup power, or if you also plan to power equipment that is intolerant of pseudo-sine wave power (such as some monitors and external drives).
BPS batteries are consumable items. Under normal conditions, a battery may be usable for between two and five years, but if you have frequent outages lasting long enough for the battery to undergo deep discharge, you may find that the battery needs to be replaced annually or more often. Many BPS units must be returned to the factory for battery replacement, which incurs very high shipping costs and leaves you without the BPS until it is returned. Better units have user-replaceable batteries, which allow you to stock a replacement battery, install it when necessary, and simply purchase another replacement. The advantage of user-replaceable batteries, both in cost and convenience, is difficult to overstate.