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

Posted on 2012-09-10
Last Modified: 2012-09-25
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?
Question by:jdana
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    There's a reasonably complete article here that I think will give you all the information you need to make the proper choices....

    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...
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    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

    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.

    Author Closing Comment

    Thanks for the great responses.

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