How to calculate PSU wattage requirements

By herr5407
Jul 12, 2007
  1. Hello all.

    I've done a little searching on these forums and have found a few posts concerning what I need but the links don't work. (or else my laptop at work sucks. which it does)

    Anyways, I was wondering if someone could explain to me the math I could do to determine wattage required for a PC. Can I take a figure off a part i install and then do some math to compare it to the PS wattage?

    This may be the easiest thing ever I just remember reading some complicated thing somewhere. Can anyone explain?
  2. halo71

    halo71 TS Rookie Posts: 1,090

  3. herr5407

    herr5407 TS Rookie Topic Starter Posts: 101

    I've seen the calculators before but I'm more interested in calculating the actual wattage myself on paper.

    The journey systems calculator is very nice though. I was just wondering if there was a forumla to follow.
  4. jobeard

    jobeard TS Ambassador Posts: 8,945   +585

    P = E * I
    P is power in watts
    E is volts
    I is current in amps
  5. herr5407

    herr5407 TS Rookie Topic Starter Posts: 101

    Also found this, not sure if it's correct. A 30% buffer seems fairly large.

    *Formula used = sum of wattage used by devices x 30% (or 1.3) buffer to be safe. And remember fans, cooling devices and anything connected via Firewire(IEEE 1394) or USB also draws power directly from the system.

  6. jobeard

    jobeard TS Ambassador Posts: 8,945   +585

    the 30% is known as a power factor which equates to 1/1.30 or 76.9% loading
    of the power supply. GOOD design says you never run anything at 100% of the
    rating as that shortents the life of the component :(
  7. herr5407

    herr5407 TS Rookie Topic Starter Posts: 101

    Ok. I was planning on building a computer in the near future and I've heard of a few people cooking PSU's from overload so I thought I would be safe and calculate it myself to get the proper wattage.

    Thanks for the information jobeard!
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