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PC PSU idle power consumption

Discussion in 'Power Electronics' started by fatman57, May 1, 2021.

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  1. fatman57


    May 27, 2013
    I am an amateur and currently building an HVAC system for my office. I am investigating fans for power usage - my question is about how much power the PSU will consume.

    I have an ad hoc lab with a 450 watt PC PSU unit I converted to run standalone. I did not make this conversion for efficiency, more to have easy access to three different voltages.

    However, during the exercise of measuring the power consumption of various fans I noticed the PSU itself was drawing a significant amount in relation to the actual power the fans were consuming. My setup was such:
    • In wall meter to measure entire power draw of PSU
    • Multimeter to measure amps and volts from PSU to fan
    • DC fan
    I found that at idle (no load) the PSU was drawing around 24 watts. The fans I am looking into range an actual draw of between 1 to 28 watts at varying voltages.

    So with that context in mind - should I expect this high draw from other power supplies (~24 watts as a baseline if the fan is not on)? I was planning to maybe switch between voltages to set the fan speed for my final setup, and I am sure if I buy for example a cheap 12v LED driver my idle consumption will be much lower - is this the type of route I should go down if I want to really lower the overall operating costs OR is my current PSU setup seriously draining too much power? It is an older PSU that I have converted.

    Thanks in advance for any replies!

    Update: I may just end up purchasing a cheap DC Motor Speed Control Driver Board off flea bay for example - but my question about power supply idle consumption still stands.

    I may also have just answered my own question - I plugged in a 5V 10Amp PSU I had and it only consumes around 4 watts at idle.
    Last edited: May 1, 2021
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    A 450 W PC PSU is designed to deliver high power at acceptable efficiency. Assuming 85 % ( a typical value) at nominal load, that means that at 450 W delivered to the PC the input power is 450 W / 0.85 = 530 W or equivalently that 530 W - 450 W = 80 W are lost internally to the power supply. This loss is inherent to the PSU . It will become smaller at lighter loads, but will still be noticeable and will reduce efficiency even more when the output power is less than 450 W. Read e.g. this discussion of the topic.
    Back to your issue: the PSU is completely overrated for the use. That is why you measure such a high loss. For best efficiency you should always use a PSU that matches the load requirements and is neither underrated nor overrated. Your own experiment with the 50 W PSU corroborates this.
  3. fatman57


    May 27, 2013
    Thanks Harold - has been a while since I have posted on this forum and you have always been a fantastic help. Much appreciated.
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