UPS Load Formula

Posted on 2011-10-27
Last Modified: 2012-05-12
  OK, I have been looking to replace a Tripp-Lite UPS that is going out but I am not a big fan of the Tripp-Lite backup we have purchased so I am planning on switching to the APC. The problem I am having is the load requirements.
  The Tripp-Lite was a 1500va/900Watts UPS and the APC I am looking at is a 1500va/865Watts. Those 35 watts should not be a problem for my system but I want to get some more information or a formula that is used to figure out what size is enough. Since I have the va and watts which one am I looking for as a unit of measurement to make sure I have enough and not too much?
  Example, one of our old PC's is has a 350w power supply and two dell monitors. The Dell web site says the monitors have a power consumption of 65w Max and 32 Typical. Can I just add the 350 with 65 twice to equal 480 and that's it? I also have a few speakers, etc but I do not have their power consumption on me at this time. When to the Va come into play?
Question by:Matthew_B
    LVL 8

    Accepted Solution

    No issue you have both the ratings for the UPS

    1. Watts: You are correct on that part.
    (You have already determined power requirements needed and UPS you are buying is sufficient)

    2: VA (Some of the UPS dont come in Watts rating just to confuse normal users) at least in India they are marketed that way...

    Simply because 1500VA maby be equal to  800W (but as a number i.e. 1500 > 800)

    To calculate VA. You need to multiply device voltage requirement with watt requirement

    i.e. Lets say


            220 x 4.7 = 1034          1034 / 1000 = 1.034 kVA

    220 is voltage and 4.7 is the amps rating on device..
    LVL 82

    Assisted Solution

    by:Dave Baldwin
    VA comes into play with 'reactive' loads that act like inductors or capacitors which include many older power supplies.  New power supplies are required to have some power factor correction to make them look more like a resistive load.

    Adding up the max power consumption is adequate for calculating your needs since it is about half of the rating for the backup supply.

    Author Comment

     So basically "Watts" is the unit of measurement that I need to be using to make sure I have a large enough power supply, correct? That is except if I am working "reactive" loads? The only things that are connected to the UPS are PCs, monitors, speakers, and a few more similar items. One UPS has a printer hooked up to it, but it is alone, except a TV.
      How close should I get to the limit of watts. What I mean is, say I have enough electronics that pull 775 watts and the backup is 800 watts. Is an 800 watt too small, and how large of a buffer should I have. I do not need the PCs to stay on for very long, because the UPS are for blinking lights, brown outs, etc.
    LVL 82

    Assisted Solution

    by:Dave Baldwin
    As long as your Maximum power dissipation doesn't exceed the power capability of the backup unit, you're ok for loading.  Equipment does not normally run at it's maximum power consumption so you should still have some margin.  Note that higher loads means less hold-up time when the power goes out.

    Author Closing Comment

    Thank you for all the help, it was GREAT!

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