Input power limitation for line interactive UPS

Discussion in 'hardware' started by Mrkvonic, Aug 14, 2011.

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  1. Mrkvonic

    Mrkvonic Linux Systems Expert

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    I have a weird question.

    Say you have a PSU that is 750W and UPS rated at 600W, line-interactive.

    Do I understand correctly that in normal mode (AC available), UPS can provide as much power as the connected source (in this case PSU) requires and ONLY when there's an outage, it will be limited to 600W output? For example, if the connected source needs 3KW, can the line-interactive UPS provide it?

    Lastly, when there is an outage in such a scenario, does this mean that if the connected source were to use more than 600W, the UPS would simply shut down or drain its battery quickly?

    Any input is appreciated ...

    Cheers,
    Mrk
     
  2. FastGame

    FastGame Registered Member

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    The UPS is 600watt @120 VAC, the PC PSU gets it rating from the 12, 5, 3.3 VDC rails combined. 600w@120 VAC is whole lot more power than 750w@12, 5, 3.3 VDC :)

    U R O K so get back to work on code :D

    edit= forgot you're not in the USA so prolly not 120 VAC....but you get the idea.
     
    Last edited: Aug 14, 2011
  3. Mrkvonic

    Mrkvonic Linux Systems Expert

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    Thanks ... was wondering whether I should shell out some more money for newer UPS now that I'm building a new desktop rig :)
    Mrk
     
  4. wat0114

    wat0114 Guest

    This I'm not so sure about. If the PS is reasonably efficient, then power in is roughly equal to power out. The 120VAC is going to draw far less current than the DC PS side. Ohm's law states: P=VxI (I=current in Amperes). Otherwise I'm not sure if the UPS will supply whatever power's needed under idle standby. Of course it's very unlikely your PS is going to work much over about 60-70% it's rated power capacity anyway.
     
  5. Bill_Bright

    Bill_Bright Registered Member

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    I think there's some confusion here.

    While the PSU plays a role, it is not the PSU that determines the load on the UPS. It is the demand placed by whatever is plugged into the power supply.

    That is, if the computer (motherboard, RAM, drives, add-in cards, drives) require 300 Watts, they will draw from the power supply 300W, regardless if the PSU is a 350W PSU or 900W PSU.

    And the PSU will draw from the wall (or UPS) 300W, plus whatever it wastes due to inefficiency. For example, a PSU rated at 80% needs to draw from the wall 125W for each 100W it supplies. So if the computer needs 300W, the PSU draws from the wall 375W - even if a 900W supply. The extra 75W is wasted in the form of heat.

    So, to determine your UPS requirement, you need to add up the demands of all your power consuming devices the UPS will protect.

    That said, if the 750W PSU does max out, your 600W UPS will (or better!!!) shutdown immediately to protect itself, and the devices it is assigned to protect. However, that capability often depends on the quality of the UPS. Like PSUs, not all are created equal.
     
    Last edited: Aug 15, 2011
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