The watts and volt-amps ratings for PCs are usually different values. In most cases,
the VA rating is never less than the watt rating and is generally larger. In fact, many de-
vices have what is called a “power factor” that indicates the percentage their watts rating
is to their VA rating. The power factor is a ratio that is expressed either as a fractional
number, like 0.8, or as a percentage, like 80 percent. The industry standard for UPS device
power factors is around 0.6 or 60 percent. Typically, a UPS device will list only its VA
rating, but you can count on its watts rating being somewhere around 60 to 80 percent of
the VA rating. The general rule of thumb for UPS sizing is that the total demand in watts
should be only 60 percent of its VA rating. The worst thing that could happen if you over-
size your UPS is that it will last longer than its load ratings.
A UPS’ VA rating indicates roughly the amount of volt-amps it can supply for about a
five-minute period. A UPS with a 300VA rating can provide 300VA for about five minutes
with a full load of around 180 watts (60 percent of 300VA). If the load is less, the UPS can
last longer. If the load is only 120 watts, the UPS may be able to provide power for 15
minutes or more.Sizing a UPS
You can size a PC by using either the amount of watts you need or the number of
volt-amps you need, whichever number you happen to have. The capacity of the UPS
should be enough to power your system for 15 minutes. This is ample time for you to shut
down the system without losing data or programs. One thing to bear in mind is that the
higher the VA rating, the more the UPS will cost. You can most definitely find a UPS that
will power your PC for an hour or more, but expect the cost to be prohibitive.
Here is a formula you can use to calculate the amount of time a UPS will support
your system:(Max. Load (Amps) x 120) + (Power (Watts) x 1.4) = Volt Amps Required
Total Volt Amps Required / Full Draw = Minimum Supply
Total Volt Amps Required / Half Draw = Nominal Supply Maximum load in amps The total draw in amps of the PC and any other
devices to be powered by the UPS. The “120” multiplier is the volts on the AC
power source.
Power supply (in watts) The watts demand of the power supply on the PC.
The 1.4 factor converts it to a VA rating.
Full draw Dividing the Total VA Required number calculated above by the
Total VA rating of the UPS should be greater than 5 minutes. However, you
never want to load a UPS this heavily.
Half draw Using a loading factor of 50 to 60 percent, the result will be a UPS
on which you can relay to supply the emergency power you need, buy
typically at least 15 to 20 minutes of standby power.(^334) PC Hardware: A Beginner’s Guide