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Help in selecting UPS unit for HP Servers

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John Chaff

Help in selecting UPS unit for HP Servers


I am trying to compare UPS units from several vendors. Most rate their output in VAs but a few rate their output in watts. Is there some conversion available so that I can compare the two values?

A. Clay Stephenson
Acclaimed Contributor

Re: Help in selecting UPS unit for HP Servers

I'll post an excerpt adapted from one of my earlier answers:

While it is true the P always equals IV there is a wrinkle and that is phase angle. VA's are used to measure apparent power while watts are used to measure actual power. In purely resistive loads (e.g. water heaters, electric strip heaters) then VA's and W's are the same; however, in real-world loads there is both a resistive component and a reactive component of the load. Reactive is a result of capacitance and/or inductance; these components store energy during part of the cycle and release it later. The vector sum of these is called impedance. In reactive loads the current (I)
is out of phase with the voltage V. The actual equation becomes P = IV Cos A . Where A is the phase angle between current and voltage. (A should be phi but I don't know how to post that.) The term Cos phi is often replaced with 'pf' power factor and a very common value for commercial power is 0.8. KVA's are often used to inflate the output of generators and UPS's. In all but the trivial case of purely resistive loads, W's will outperform VA's of the same rating everytime.

Specifically, suppose vendor A quotes a 100 KW unit and vendor B quotes a 100 KVA unit. If we assume a typical 0.8 pf, then A is actually a 125KVA unit.

I suppose the best analogy (close but not exact) is that calling VA's and W's the same is like calling speed and velocity the same thing.

Of course, your HP salesperson would surely tell you that HP makes excellent UPS's for HP boxes. You should look for those that allow easy status querying so that you can craft shutdown scripts --- although I never both with shutdown scripts.
If it ain't broke, I can fix that.