How many watts does a UPS draw when powering from a generator

Posted on 2008-01-30
Medium Priority
Last Modified: 2013-12-11
I am in South Africa 220V power, with constant power outages I have purchased a small generator 2500 Watts just to run computers.  I have a 650VA UPS & a 1000VA UPS the manual says they use 390W and 600W respectively.  I just want to confirm that they will infact draw 390W and 600W when powered by a generator as this will influence what equipment I run off my generator.
Question by:schwegums
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Accepted Solution

nobus earned 1800 total points
ID: 20775749
the easies t way is to measure it, use a wattmeter, or Amps meter and calculate the power used
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Assisted Solution

by:Gary Case
Gary Case earned 200 total points
ID: 20775795
I believe your UPS units probably saw they can PROVIDE 390W and 600W respectively.   They will actually draw an apparent load of 650VA and 1000VA, respectively, but have a power factor of about 60%.   The power factor determines the actual relationship between the "apparent power" (VA) and "real power" (W).   If the power factor was 1, then Ohm's law of W = VA would apply (but it does not in this case).

This means your source (the generator) needs to be able to supply the full apparent power of 1650VA.   Most generators are rated for both KVA and KW output --> be sure you know which you are dealing with.   The "safe" assumption is that you need 1650W of the generator's 2500W output.

Note that this is only drawn under maximum load ... it would only be drawn if you're loading the UPS at 100% of its stated capacity (not likely and certainly not a good idea).   If you know you never draw full load from the UPS's, you can reasonably assume a somewhat lower load when computing what you can power from the generator.

Author Comment

ID: 20823882
Thank you for your help.  I bought an amp meter and it did the trick.
LVL 93

Expert Comment

ID: 20823984
then please close the question
LVL 70

Expert Comment

by:Gary Case
ID: 20824025
Measuring the actual draw is always the best way :-)  => Don't forget to close the question.

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