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I've got some M-Audio AV40s at home, plugged into a power strip that includes a mac mini and an LCD monitor. If I plug the power strip into a power meter (like a kill-a-watt, except I live in NZ so I have a different brand that can handle 240V), I might observe the following:

  • Speakers off: 20W
  • Speakers on: 15W

Why does turning the speakers on save me five watts?

The meter also records the power factor as 100 in the first case and 95 in the second, but I don't know what that means.

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What are the readings if the speakers aren't plugged in at all? –  retracile Aug 18 '09 at 2:54
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agreed... maybe your internal speakers are now turned off? –  RiddlerDev Aug 18 '09 at 2:57
    
@retracile: Same as when they're turned off. @IPX Ares: This is when the mac is asleep, so I wouldn't think the internal speakers are doing anything. –  John Fouhy Aug 18 '09 at 3:00
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You should have people pay you to turn your speakers on, and use your 5 watts. –  Noam Gal Aug 18 '09 at 6:32
    
Does your killawatt read zero for other appliances that are turned off? If not, faulty unit. –  Kent Boogaart Aug 18 '09 at 15:55
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4 Answers 4

As an Electrician, and without the devices in my hands, I would say the reason for the drop in consumption is down to power factor correction. Fluorescent lamps have a capacitor to counteract the inductive choke (transformer).

It's been 25 years since I studied this, but I think 90 decrees is the perfect correction factor. Therefore the drop from 100 to 95 is half way there. Please note: I really cannot remember fully and I could be completely wrong about the 90 degrees.

I recommend leaving the speakers plugged in and help save the planet. :-)

Hope this helps.

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Since this is fresh in my mind from my EE courses, here's the short explanation: You take standard Cartesian coordinate system and put active (resistive) power consumption on Y axis and reactive (capacitative or inductive) power consumption on X axis. Then you plot a line from center of the coordinate system to the position marked by X and Y coordinates. The length of the line is apparent power which is shown by the meter and the angle between the line and X axes is marked φ. Power factor is calculated as cos(φ) and ranges between 0 and 1. –  AndrejaKo Jan 2 '11 at 11:43
    
When φ is 0° or 180°, we have purely reactive load which is either capacitative or inductive. When φ is 90°, we have purely resistive load and the reactive power is zero. Since power factor ranges between one and zero, it is often expressed in percentages. So power factor of 100 means that cos(φ) is 1 and that means that φ is 90°, while when power factor is 0, it means that cos(φ) is 0 and that φ is 0° or 180°. –  AndrejaKo Jan 2 '11 at 11:46
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The difference may have to do with that power factor change. Those well acquainted with Ohm's Law for DC circuits (including me) are easily confused by the complexities of A/C loads. In this case, your 20W of drain is more efficient at PF 100% than your 15W at 95%.

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I'd say it's unlikely you're truly using less power when they're plugged in than when they're not.

You've commented that you're using the same power when they're off, or not plugged in at all, but the number goes down when you turn them on.

My initial thoughts are that your power meter isn't super accurate near the zero end. If it's anything like the Kill-a-watt units, they claim up to 0.2% accuracy (1% typical), but don't provide a minimum reading.For example, the 110V version says it's accurate to 0.2%, but doesn't say over what range, 0.2% of full scale is about 4 Watt, 1% is 18W.

Things to try.
1. Load the powerboard up with something of at least a couple of hundred fairly steady watts, then plug the speakers in and out and see if the change is the same.
2. Do the same measurements with the audio source connected and disconnected, to eliminate the loading issue.

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Its probbaly an accuracy issue with the meter. Its not uncommon for cheaper personal type power meters to be somewhat inaccurate on the lower end of usage

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