Hardware Reference
In-Depth Information
maximum power draw in watts, or sometimes in just volts and amperes. If only voltage and amperage
are listed, multiply the two figures to calculate the wattage.
As an example, if the documentation for a system indicates that the computer can require as much as
120V at a maximum current draw of 5 amps, the maximum power the system can draw is about 550
watts. The system should never draw any more power than that; if it does, a 5-amp fuse in the power
supply will blow. This type of system usually draws an average of 200 to 300 watts. However, to be
safe when you make calculations for UPS capacity, be conservative; use the 550-watt figure. Adding
an LCD monitor that draws 50 watts brings the total to 600 watts or more. Therefore, to run two fully
loaded systems (including monitors), you'd need a 1,200-watt UPS. A UPS of that capacity or greater
normally costs several hundred dollars. Unfortunately, that is what the best level of protection costs.
Most companies can justify this type of expense only for critical-use PCs, such as network servers.
Note
The highest-capacity UPS sold for use with a conventional 15-amp outlet is about 1,400 watts.
If it's any higher, you risk tripping a 15-amp circuit when the battery is charging heavily and
the inverter is drawing maximum current.
In addition to the total available output power (wattage), several other factors can distinguish one
UPS from another. The addition of a ferroresonant transformer improves a unit's power conditioning
and buffering capabilities. Good units also have an inverter that produces a true sine wave output; the
cheaper ones might generate a square wave. A square wave is an approximation of a sine wave with
abrupt up-and-down voltage transitions. The abrupt transitions of a square wave are not compatible
with some computer equipment power supplies. Be sure that the UPS you purchase produces power
that is compatible with your computer equipment. Every unit has a specification for how long it can
sustain output at the rated level. If your systems draw less than the rated level, you have some
additional time.
Caution
Be careful! Most UPS systems are not designed for you to sit and compute for hours through an
electrical blackout. They are designed to provide power only to essential components and to
remain operating long enough to allow for an orderly shutdown. You pay a large amount for
units that provide power for more than 15 minutes or so. At some point, it becomes more cost-
effective to buy a generator than to keep investing in extended life for a UPS.
Some of the many sources of power protection equipment include American Power Conversion
(APC) and Tripp Lite. These companies sell a variety of UPS, SPS, line protector, and surge
protector products.
Caution
Don't connect a laser printer to a backed-up socket in any SPS or UPS unit. Such printers are
electrically noisy and have widely varying current draws. This can be hard on the inverter in
an SPS or a UPS and frequently cause the inverter to fail or detect an overload and shut down.
Either case means that your system will lose power, too.
