Hi All,
After figuring out that the root cause of my recent system failure was
probably a faulty UPS unit, I set about trying to better understand
those magical boxes. I will probably never know if the UPS failed as a
result of an electrical storm or if the damage was operator/user error.
It took a lot of reading to finally wrap my brain around volt amperes,
watts, automatic voltage regulation and pure sine-wave output. (Some
times I am extremely dense, just ask Ron) A friend asked me to share
the information I collected on how to calculate how much backup power a
computer setup actually needs. For the benefit of other non-electrical
types, I am passing on the information.
UPS selection for Dummies:
The rating on a backup unit is usually part of its name. For example an
APC 650 UPS (American Power Conversion brand) is rated for 650VA . With
others you will have to look on the box or in the paperwork, to find the
VA rating.
Many UPS units have both battery backup outlets and surge only outlets.
In theory, you only want to "load" the battery backup outlets of a UPS
to about 50% of the VA rating. If you load it more than around 65%, you
will not have adequate time to shut down the computer in the event of a
power blackout. Most UPS manufacturers warn against plugging a surge
strip into the battery backed up outlets. So, if you need a few more
plugs for your less sensitive equipment, plug your power strip into the
"surge protection only" outlets of the UPS.
The ratings listed in the advertising, and on the packaging, for UPS
units are bull droppings. It will say something like 75 minutes of run
time. However, if you do the math, or read the fine print, that "run
time" is with a 30% or lower load.
To figure out how many VA (volt ampere) your equipment uses you need to
check the label (usually a metal tag) on the back of a computer or
monitor. The information is on the power supply of things like zip
drives, printers, scanners, etc. You want to use the input numbers, not
the output of the power adapter. The information is not consistent in
how it is listed. You will have to do some math to convert everything
to VA. Some things, like scanners will list a range (as they pull a
considerable amount of power when scanning). Use the higher number in
the range.
VA (volt ampere) - if it list the VA, use that number
mA (milliampere) - multiply mA x 1000 x 120 = VA
A (ampere) - multiply A x 120 = VA
W (watt) - multiply watts x 1.4 = VA
Add up the total VA and compare that to the number rating on your
Uninterrupted Power Supply box. As mentioned above, you are usually
safe if you do not exceed 65% of the total VA listed. The numbers
listed on the equipment are almost always slightly overstated by the
equipment manufacturers.
I suspect there are a number of individuals on this List who can
elaborate on this information. Elaboration anyone?
Nancy
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