PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

27.4 Choosing a BPS

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.