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Shopping UPS Units - How do I calculate how long it will last?

I'm shopping a UPS unit for a generic device. Let's say the device pulls X amps. Is there a quick-and-dirty calculation that allows me to calculate how long a given UPS will supply my device?  

I'm seeing units associated with UPS units of "VA" (volt-amps) and Watts. I'm a little confused, isn't a Volt-Amp equal to a Watt?
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jdana
Asked:
jdana
4 Solutions
 
Scott CSenior Systems EnginerCommented:
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btetlow-expertCommented:
There's a reasonably complete article here that I think will give you all the information you need to make the proper choices....

http://www.ehow.com/how_7429261_calculate-ups-run-time.html

Also -- VOLT (times) AMP = WATTS  

An online calculator -- just leave the part you want to know blank, and it'll tell you the missing information...  

http://www.supercircuits.com/resources/tools/Volts-Watts-Amps-Converter
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Scott CSenior Systems EnginerCommented:
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andyalderCommented:
Volts * Amps equal watts with DC but it is more complicated with AC since peak current is not necessarily drawn at peak volts, there are some drawings explaining it at http://en.wikipedia.org/wiki/Power_factor

You probably have an 80+ PSU if it is a modern computer so power factor is greater than 0.9, and 1000VA=900W at 0.9 power factor. If you do have a modern computer or similar PSU you also have to avoid UPSs which have simulated sine wave outputs aince they just crash all the time on them, for example APC Back-UPS are no use but APC Smart-UPS are OK.
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jdanaAuthor Commented:
Thanks for the great responses.
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