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  1. #1
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    Going from a 85% to a 90% effecient PSU worth it?

    How much actual wattage drop from the wall with a given PC drawing 100 watts using a old 80plus PSU (before the current standards were published) going to a 85% efficient PS and then to a 90% efficient PSU rated around 600w (for example)? IOW's when measuring the AC current at the wall, using a 600w supply as the example, how much less wattage could I expect this to draw what the AC side?
    Last edited by videobruce; 08-06-2013 at 01:26 PM. Reason: clarified text
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  2. #2
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    It depends on the load your system is drawing, but think of it this way.

    Assuming 500W system draw (CPU/Mobo/GPU etc)

    @85% Efficiency: 500W/0.85 = 588W Drawn from the wall (aka 88W in wasted energy)

    @90% Efficiency: 500W/0.90 = 555W Drawn from the wall

    So, going from 85% to 90% nets you 33W in saved energy at the wall. Of course, split your starting load in half (500 -> 250W) and you're now saving ~16W at the wall.

    If its "worth" it depends a lot on your actual load, as well as the cost of electricity at your location. A place with expensive electricity is going to have a shorter payback period. If you're putting a really big load on your PSU for long periods of time (e.g. Folding@Home), then the watts start to add up a lot faster than if you're doing desktop tasks and idling (<100W). By my estimation, it often doesn't pay to get a new power supply solely for an efficiency benefit. If you've got a crummy low efficiency PSU, it might be worth it to upgrade to a quality high efficiency PSU, but if they're both of equal quality and one just has better efficiency, then I wouldn't bother. In other words, efficiency is definitely a piece of the puzzle when I consider PSU purchases, but its not usually the sole reason I upgrade.


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  3. #3
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    That's a difficult, perhaps even impossible question to answer in practice (without actually measuring the difference), as it depends what the actual system you are running uses and as the efficiency of the PSU's changes somewhat with the wattage output also. It is perfectly possible that the PSU rated 85% is more efficient at idle loads.

    But if we assume that the numbers hold true from 0 to 600W output:
    In theory the max difference would be 40watts, but that is a situation that never exists (both PSUs loaded at 100%, which noone would ever even recommend). At idle let's assume your system uses 90Watts, in which case the PSU rated at 90% efficiency would actually use 100W and the 85% PSU a bit under 106, so ~6W. Under load (~300W) the difference would be around 20W.

  4. #4
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    I clarified my OP.

    Activate: AMD;
    Those figures are the output, not the input. PC's don't consume 600 watts at 120VAC (at least not yet). My current buoild from 7 years ago draws around 100watts at 120VAC.
    When not in use, turn off the juice!

  5. #5
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    Quote Originally Posted by videobruce View Post
    I clarified my OP.

    Activate: AMD;
    Those figures are the output, not the input. PC's don't consume 600 watts at 120VAC (at least not yet). My current buoild from 7 years ago draws around 100watts at 120VAC.
    I think I addressed that to some extent in my post, though not explicitly. I was merely giving an example of a 500W power draw from the components themselves, as in the load that the PSU would need to deliver from its own output. This power draw would theoretically remain fixed between power supplies and then my calculated values would be the power drawn from the wall at the given efficiency (i.e. the amount of power the PSU would need to deliver the 500W to the components). You need to size your PSU at the high end for max consumption, so my 500W number is not totally unreasonable for a high-end setup at 100% load. That said, picking the right numbers for efficiency comparisons is a little trickier. Do you leave your PC running 24/7 at full load or does it run for 20 minutes a day writing e-mails? Those two scenarios are at complete opposite ends of the spectrum when it comes to power consumption and have very different implications for deciding whether or not to upgrade solely for efficiency gains.

    So to answer your question: if you're drawing 100W and don't do anything intensive that requires extended periods of full-load, then the payback period for a new 90% efficient PSU vs the old 85% one is going to be really really long.

    Not to make too big an issue of it, but you're wrong if you believe that PC's don't consume 600W under full load. Slap 2 high-end video cards in a system and you can easily get to 650W or more under full load without any kind of overclocking at all. My rig will draw 400W at load, probably more (i7-950 + AMD 7970GPU), though it is an older and more power hungry CPU/MB combo.


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  6. #6
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    Quote Originally Posted by Activate: AMD View Post
    Not to make too big an issue of it, but you're wrong if you believe that PC's don't consume 600W under full load. Slap 2 high-end video cards in a system and you can easily get to 650W or more under full load without any kind of overclocking at all. My rig will draw 400W at load, probably more (i7-950 + AMD 7970GPU), though it is an older and more power hungry CPU/MB combo.
    You are correct, naturally, with the SLI-setup power usage under load. Another way to drastically increase the power consumption is to OC your CPU. Still, you really have to try to stress your system, e.g. I'd bet your system - assuming no OC on the CPU/GPU - under an intensive session of BF3 actually does peak around the 300W mark (+/- some depending on further details of the system, perhaps closer and even topping the 400W you mention, especially if your PSU isn't too efficient near that wattage).

    Point being: a halfway decent 600W PSU should handle a single CPU, single Gfx-card system easily.

  7. #7
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    I don't OC and have no need for multiple video cards. Frankly, I don't know why there is a need. What has dual engines in any vehicle other than some industrial or military equipment? And no. I'm not talking about any watercraft. If a video card can deal with software, then the software needs to be changed.

    To me, 100 watts is excessive. My Laptop draws 1/3 of that and that isn't counting the PC's monitor.
    When not in use, turn off the juice!

  8. #8
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    Quote Originally Posted by videobruce View Post
    I don't OC and have no need for multiple video cards. Frankly, I don't know why there is a need. What has dual engines in any vehicle other than some industrial or military equipment? And no. I'm not talking about any watercraft. If a video card can deal with software, then the software needs to be changed.

    To me, 100 watts is excessive. My Laptop draws 1/3 of that and that isn't counting the PC's monitor.
    Thats just your opinion. There are plenty of high-end applications that require multiple CPU's or GPU's because the hardware hasn't caught up to the requirements for performance. I don't really see how this is relevant to your OP though.


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  9. #9
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    It is when I'm not looking for that much capacity.
    When not in use, turn off the juice!

  10. #10
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    Re: Going from a 85% to a 90% effecient PSU worth it?

    Quote Originally Posted by videobruce View Post
    It is when I'm not looking for that much capacity.
    Right, and I gave you the calculations to figure it out as well as my thoughts on the subject. You can expect to save a few watts on a load that small.. <10W. i.e. small enough to make the payback time excessively long


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