Power consumption question

Discussion in 'Power Supplies' started by infra1, Apr 6, 2010.

  1. infra1

    infra1 n00b

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    Hey guys,

    What's a good app (or method) in determining how much power your computer draws (including monitor(s)), which then translates it to a monthly monetary figure?


    Cheers!
     
  2. Dangman

    Dangman Ninja Editor SuperMod

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    Be wary of the KAW figures. They're inaccurate when used with most modern PSUs with APFC.
     
  3. infra1

    infra1 n00b

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    I'd like to read about any estimates you guys have of your own systems. How much do you think she's running your electricity bill per month? I'm looking for a ballpark figure to be honest. Maybe someone might have a comparable system.
     
  4. NoForgiveness

    NoForgiveness Gawd

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    Your system consumes probably under 200watts when idle and sees peak load use of just over 400 or so watts.
     
  5. infra1

    infra1 n00b

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    I'm not exactly an electrician pro, but how can I translate this to a rough estimate in terms of cost to my electrical bill?
     
  6. Zero82z

    Zero82z [H]ard as it Gets

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    Multiply wattage by the number of hours your PC is on within a billing period. Multiply that by your cost per kW*h. That will give you a rough estimate of how much money it will cost you.
     
  7. infra1

    infra1 n00b

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    Cheers! This will be interesting to know :)
     
  8. Talonz

    Talonz 2[H]4U

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    How much of a factor are we talking about? Has anyone used a professional meter vs the KAW for a APFC PC load?

    I understand that the values will be off by a few percent due to APFC, but +/- 5% is good enough for what most of us are using it for (ballpark figures and relative wattage estimates). For example, my system in sig idles at just 88 watts minimum, which seems in line with reviews.
     
  9. Dangman

    Dangman Ninja Editor SuperMod

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    Yes:

    Here's Paul Johnson's post, PSU reviewer of HardOCP.com, about the inaccuracy of the Kill-A-Watt:
    http://hardforum.com/showpost.php?p=1032190998&postcount=7

    In addition, three other PSU experts backs up Paul Johnson's statement:
    Oklahoma Wolf of JonnyGuru.com:
    http://hardforum.com/showpost.php?p=1034843536&postcount=21

    JonnyGuru of JonnyGuru.com himself as well as the senior PSU engineer over at BFG acknowledge the inaccuracies of a KAW (Post #7 in regards to Post #2):
    http://www.jonnyguru.com/forums/showthread.php?t=5977

    Redbeard of Corsair also acknowledges the inaccuracies:
    http://hardforum.com/showpost.php?p=1032811067&postcount=22

    So +/- 5% can't be expected.
     
  10. keenan

    keenan 2[H]4U

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    Given the specifications on the device, I wouldn't expect it to do much worse than that, and the physics of it should mean that APFC makes it more accurate, not less. However there have been some reports of it reading wildly low, on the order of 20% or more. I'm not convinced that there isn't something else weird going on in those situations, but without more information I figure it's a case of YMMV. Anyone know of a PSU that is known to cause this effect? I'm actually pretty curious to look into this myself.
     
  11. Talonz

    Talonz 2[H]4U

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    I read through those but mostly they are concerned with the KAW overestimating efficiency when there's APFC. None of those statements show a big innaccuracy. As I understand it, Paul Johnson's post shows the KAW reading 215w when 219 true watts are being provided and 353 VA is the apparent power. This would put the KAW within 2% in that test, which is good enough for most of us.
     
  12. Dangman

    Dangman Ninja Editor SuperMod

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    Ummm, you misunderstood Paul Johnson's post and KAW does not estimate efficiency. Read the post again:

    The PSU is outputting 219W on the DC side. The KAW is saying that the PSU is drawing 215W from the wall on the AC side. That's 102% efficiency. The Brand Digital Power Meter shows the true figure of 353W being drawn from the wall on the AC side. So in other words, when the PSU is outputting 219W, it's drawing 353W from the wall. As such, the KAW was off by a full 40%. Those statements show that other PSU experts have found and know that KAW are inaccurate.
     
  13. keenan

    keenan 2[H]4U

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    We don't really know what his setup is showing since the measurement setup is never described as far as I can tell.

    Some obvious oddities:

    - Brand meter shows a power factor of 0.00. For an APFC supply this is obviously...strange...to say the least. PF should be very close to 1. A 0 power factor is the result you'd get with a purely inductive or purely capacitive load. Even a non-PFC supply shouldn't be less than 0.4 or so.
    - Variac is in the circuit, yet no apparent measurement of the output voltage is shown (and he blindly multiplies by 120V to get wattage)
    - Irms that the Brand is measuring is a component of apparent power anyway, which is not what we're trying to measure anyway, the real power is the relevant measurement, and the Brand can measure that, but this measurement is not shown. This is what Talonz points out.

    Edit: If the power factor were actually close to 1 though, as it should be, apparent power = real power, and this Irms measurement would reflect the real power. However the KAW, even if it's very poorly designed, shouldn't have problems measuring this with reasonable accuracy, even a pair of cheapie multimeters without True RMS can do the job. It just doesn't make sense, and I'd need to see some real investigation into the mechanism behind the inaccuracy before I'll buy into it being a real problem.
     
    Last edited: Apr 7, 2010
  14. Talonz

    Talonz 2[H]4U

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    Only the current is shown, so multiplying it by the voltage gives us the VA, not the wattage (which is modified by the power factor).

    VA * PF = Watts

    So the SM-8800 (connected APFC power supply) measures 219 watts, KAW is reporting 215 watts drawn by the PSU, and we're seeing an AC apparent wattage of 353 watts. The difference between 353 and 219 should be due to power factor and AC-DC conversion. What's the actual setup?

    Edit: Good point about the PF being 0.00 on the power meter, that's definitely not right.
    Edit2: Well there's also a transformer in the loop, does it go: Wall -> Brand Power Meter -> Variac -> KAW -> Load? That would change a lot, in fact we would be measuring totally different things!
     
    Last edited: Apr 7, 2010
  15. Zero82z

    Zero82z [H]ard as it Gets

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    The SM-8800 is not measuring 219W. 219W is the load that the SM-8800 is putting on the PSU. Therefore, the PSU must be drawing more than that from the wall. However, the Kill-A-Watt is measuring a total draw of 215W, which is obviously incorrect since it is less than the load the PSU is under.
     
  16. Dangman

    Dangman Ninja Editor SuperMod

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    This discussion about the KAW's inaccuracy has come up before on another forum 3 years ago:
    http://www.jonnyguru.com/forums/showthread.php?t=3102
    http://www.jonnyguru.com/forums/showthread.php?t=2695
    http://www.jonnyguru.com/forums/showthread.php?t=2284

    From what I can gather, theoretically, APFC shouldn't be causing the inaccurate load measurement but from the experiences of PSU experts Paul Johnson (HardOCP), JonnyGURU, OklahomaWolf (JonnyGuru), Rebeard (Corsair), and apparently SKYMTL (HardwareCanucks) the KAW is showing inaccurate numbers with PSUs that have APFC. Apparently, judging from the above discussions, no one knows why exactly the inaccuracy is occurring.

    EDIT: AFAIK, no one has actually tried investigating why the KAW inaccuracy occurs. Some theories here and there but no solid conclusion to the matter.
    EDIT: As for the testing setup shown in the pics, refer to this:
    http://www.hardocp.com/article/2007/02/25/hard_look_at_power_supplies/3

    A tad outdated since it's missing the Brand Power Meter and Extech analyzer.
     
    Last edited: Apr 7, 2010
  17. keenan

    keenan 2[H]4U

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    If some loads cause this effect but not others, characterizing those loads is a start towards figuring out why this happens. Until then any measurement should be in question, because two measurements that don't agree from two instruments of which neither is particularly well specified just tell you that you don't know what the correct value is.

    Unless there's a serious bug in the KAW firmware, and I think such a problem would show up with any load, there doesn't seem to be any reason for any errors of large magnitude. As Kvar points out in one of those threads, reasonable measurement errors (since these are hardly precision tools) may lead to small discrepancies and slightly 'illegal' values. They might not even be as good as the claimed 2% or whatever it is, even for loads with quite a good PF. But an error of large magnitude indicates that there's a deeper problem somewhere, either in the KAW firmware itself (but as I said, I have a hard time thinking of a class of bug that would apply only to certain APFC supplies and not others, or non-PFC loads) or in the measurement methodology.

    Basically, nothing that's been shown makes it clear where the uncertainty is actually coming from, just that uncertainty exists. It could be factors entirely unrelated to the KAW, or it could be the other measurements that are in error for some reason. I still haven't seen any one of the many threads I've read on this actually say which device is being measured.