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Basic question about PSU Wattage and Efficiency

I have a simple question about PSU efficiency (I'm trying to settle a debate with a friend). Let's say I buy a theoretical, totally reliable, able to sustain max load indefinitely power supply rated at 1000W on the box, with an guaranteed 80% efficiency at all loads and temperatures. Which statement would be correct:

1: When PC is drawing all the power it can from the PSU, it will receive 1000W and a Kill-A-Watt would measure 1250W at the wall.
2: When PC is drawing all the power it can from the PSU, it will receive only 800W and a Kill-A-Watt would measure 1000W at the wall.

A rephrase of this question might be "Is the power rating of a PSU the max power it will draw from the wall or the max power is can supply to the PC components?"

Sorry if this has been answered before, I am having a hard time finding this information. :)

Also, a related question, "Can an 90% efficiency 900W PSU deliver more power to a PC than a 75% efficient 1000W PSU?"
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  1. Best answer
    This is the correct answer:

    1: When PC is drawing all the power it can from the PSU, it will receive 1000W and a Kill-A-Watt would measure 1250W at the wall.


    A rephrase of this question might be "Is the power rating of a PSU the max power it will draw from the wall or the max power is can supply to the PC components?"

    The power supply is rated by what it can deliver to your computer components........


    Also, a related question, "Can an 90% efficiency 900W PSU deliver more power to a PC than a 75% efficient 1000W PSU?"

    NO. The 900watt supply will deliver 900watts to your PC components. The 1000watt will deliver 1000watts.

    NOTE: The 75% efficient supply will just give you a higher electric utility bill to pay....

    The goal in buying the higher efficiency supplies are:
    - higher efficiency means less heat generated by the supply
    - and, lower cost on your utilities bill
  2. a good psu will deliver its rated capacity


    if efficiency is 80% the loss will be 20% so your draw at the wall will be 120% of the power drawn from your psu
  3. Ok, looks like my buddy was wrong! Thanks.
  4. Best answer selected by avihappy.
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