Does Power Supply consume watts all the time?

Discussion in 'Computer Information' started by dwkerschen@gmail.com, Jun 14, 2007.

  1. Guest

    Does Power Supply consume watts all the time?

    I am having a PC built for me (middle of the road... just do not want
    bloat-ware) and I have a choice of how big a power supply I can
    select.

    I do not want to waste energy with a bigger PS (I leave my PC on/in
    sleep 24x7) if it just sits there and consumes 500 watts when I know
    that a smaller one would be fine.

    Thx
    Dave-in-Denver
    , Jun 14, 2007
    #1
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  2. Paul Guest

    wrote:
    > Does Power Supply consume watts all the time?
    >
    > I am having a PC built for me (middle of the road... just do not want
    > bloat-ware) and I have a choice of how big a power supply I can
    > select.
    >
    > I do not want to waste energy with a bigger PS (I leave my PC on/in
    > sleep 24x7) if it just sits there and consumes 500 watts when I know
    > that a smaller one would be fine.
    >
    > Thx
    > Dave-in-Denver
    >


    The power supply has a number printed on the side. Say it is "500W".

    The 500W is the maximum amount of power that can be drawn from
    the DC outputs.

    For an average build, when the computer is sitting there, it might
    draw 100W - 150W. It doesn't matter whether a 500W supply is used,
    or a 750W supply is used, 100W to 150W is what is coming from the
    wall.

    When you start a game, and you have some fancy video cards, the power
    drawn would increase to maybe 300W. So gaming adds more to your
    power bill, than when the computer is just sitting there. Again,
    it is the load the components make, that draws the power. The
    500W number is the maximum output.

    When you put the computer to sleep (S3 Suspend to RAM), the +5VSB
    output continues to run. On my computer, it uses +5VSB at 0.40 amps
    or about 2 watts. Some computers will draw more. The power is used
    to maintain DRAM refresh, so the DRAM keeps all the memory cells
    charged with their 1's and 0's, by using some +5VSB as a power
    source.

    Another issue, is the efficiency of the supply.

    Say the computer is sitting there idle. Say the outputs are providing
    exactly 100W. To just make up some numbers, 12V @ 5A, 5V @ 4A, 3.3V @ 6A.
    Those are the DC outputs. The power comes from the A.C. wall plug.
    The power supply has a "conversion efficiency", and that is how
    good a job it does of turning the A.C. at 120 or 240V, into DC.

    A cheap supply might be rated for 68% efficiency. To make 100 watts,
    it needs 100W/0.68 = 147W from the A.C. wall plug. The 47W difference,
    is waste heat. 147W comes from the power company. 100W goes to the
    motherboard. 47W goes into the room as heat.

    There are more expensive supplies, that are 80% or even higher
    efficiency. Using the same 100W output load from the motherboard,
    the wall power is 100W / 0.80 = 125W. Notice how the more efficient
    supply, uses 22W less A.C., and if you feel the exhaust air coming
    from the more efficient supply, it is cooler. They might even be
    able to reduce the fan speed, use lighter heatsinks, and so on,
    inside the PSU.

    It takes a long time, to pay back that 22W saving. If my power is
    $0.07 per kilowatt hour, after 45 hours I would save a kilowatt hour
    of electricity. Which means I'm saving $0.07 about every two days, or
    about $12.78 per year. If the power supply is $70 more than a less
    efficient model, it will take 5.5 years to pay back the cost of
    the more efficient supply, via the power saved.

    That is not the main reason for doing it. The main reason, is
    your computer room won't warm up quite as much, with a more
    efficient supply. I guess it all depends on when you can run
    air conditioning, as to how much of a factor this is.

    Paul
    Paul, Jun 14, 2007
    #2
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  3. Guest

    >
    > The power supply has a number printed on the side. Say it is "500W".
    >
    > The 500W is the maximum amount of power that can be drawn from
    > the DC outputs.
    >
    > For an average build, when the computer is sitting there, it might
    > draw 100W - 150W. It doesn't matter whether a 500W supply is used,
    > or a 750W supply is used, 100W to 150W is what is coming from the
    > wall.
    >
    > When you start a game, and you have some fancy video cards, the power
    > drawn would increase to maybe 300W. So gaming adds more to your
    > power bill, than when the computer is just sitting there. Again,
    > it is the load the components make, that draws the power. The
    > 500W number is the maximum output.
    >
    > When you put the computer to sleep (S3 Suspend to RAM), the +5VSB
    > output continues to run. On my computer, it uses +5VSB at 0.40 amps
    > or about 2 watts. Some computers will draw more. The power is used
    > to maintain DRAM refresh, so the DRAM keeps all the memory cells
    > charged with their 1's and 0's, by using some +5VSB as a power
    > source.
    >
    > Another issue, is the efficiency of the supply.
    >
    > Say the computer is sitting there idle. Say the outputs are providing
    > exactly 100W. To just make up some numbers, 12V @ 5A, 5V @ 4A, 3.3V @ 6A.
    > Those are the DC outputs. The power comes from the A.C. wall plug.
    > The power supply has a "conversion efficiency", and that is how
    > good a job it does of turning the A.C. at 120 or 240V, into DC.
    >
    > A cheap supply might be rated for 68% efficiency. To make 100 watts,
    > it needs 100W/0.68 = 147W from the A.C. wall plug. The 47W difference,
    > is waste heat. 147W comes from the power company. 100W goes to the
    > motherboard. 47W goes into the room as heat.
    >
    > There are more expensive supplies, that are 80% or even higher
    > efficiency. Using the same 100W output load from the motherboard,
    > the wall power is 100W / 0.80 = 125W. Notice how the more efficient
    > supply, uses 22W less A.C., and if you feel the exhaust air coming
    > from the more efficient supply, it is cooler. They might even be
    > able to reduce the fan speed, use lighter heatsinks, and so on,
    > inside the PSU.
    >
    > It takes a long time, to pay back that 22W saving. If my power is
    > $0.07 per kilowatt hour, after 45 hours I would save a kilowatt hour
    > of electricity. Which means I'm saving $0.07 about every two days, or
    > about $12.78 per year. If the power supply is $70 more than a less
    > efficient model, it will take 5.5 years to pay back the cost of
    > the more efficient supply, via the power saved.
    >
    > That is not the main reason for doing it. The main reason, is
    > your computer room won't warm up quite as much, with a more
    > efficient supply. I guess it all depends on when you can run
    > air conditioning, as to how much of a factor this is.
    >
    > Paul


    Holy Cow!!!

    Man... do I love communicating with people who are clear and know what
    they are talking about!!!

    Thank you Paul!!!

    Dave-in-Denver
    , Jun 14, 2007
    #3
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