About VAs vs. Watts (a message I posted recently on another list):

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The power rating on most UPSs is in VA, not W, because the UPS has a
limited capability to cope with high cos phi situations. So Adding up the
Watts on the connected appliances may not be right, if they have high (and
not compensating each other) cos phi values. Typical items that may upset
your calculation are fluorescent lights (which have inductive lag from the
ballast choke) and squirrel cage motor fans (which have inductive lag).
Modern monitors and computer power supplies do not cause problems
however, although they have a small capacitive lead.

As an example:

fluorescent light = 40W at cos phi = 0.8 will take 40W but 40/0.8 = 50VA.
A 220V,50W fan with cos phi=0.6 will take 50/0.6 = 83VA. As you can see
the VAs add up faster than the W's. You want to calculate the VAs to see
if the UPS will carry the load. For most normal situations adding the
Watts and leaving a generous safety margin (30% or so for 2-3 computers
and 2-3 monitors) will be enough. For this consider the UPS power rating
in VA, as being Watts.

Peter

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