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The title pretty much sums it up... Hyperbole aside, how much actual power is consumed by a typical wall wart when the device it's attached to isn't powered on? And are there designs for wall warts that minimize or eliminate this sort of parasitic load?

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I kept reading "wall wart" as "wal mart" and was very confused. –  JD Isaacks Nov 3 '10 at 16:56
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5 Answers

up vote 12 down vote accepted

About 1 billion percent. No useful work is being done except to heat your house, and energy is being dissipated which would not be dissipated if the wall wart was not plugged in. The actual quantity is small, so I wouldn't worry about it too much if you're concerned about your electricity bill - A few cell phone chargers left in the wall 24-7 won't make a dent. For some math, if a device is just barely CEC certified (0.5W no-load dissipation) and left in the wall unused all month, you have 730 hours (in one month) * 0.5W * 0.001 W/kW = 0.365kWh [kilowatt hours] and electricity costs of, say, $0.20/kWh, and you have a cost of $0.07 Find a dime, pay the bill. Compare that to, say, your 73W fridge, which will be around $10/month.

If, however, you have a lot of wall warts, or the small losses are significant to you, look at switch-mode power supplies. They're more expensive, but more efficient both at no-load conditions and during use, and switch-mode supplies will actually regulate, as opposed to transformer-based designs which require further regulation circuitry. You can tell what kind a converter is just by the weight - A switch-mode supply will have just a lightweight circuit board with some electronics inside, while a traditional wall wart has a big, heavy transformer. I've used CUI switch-mode PSUs before, they're a good company with a nice line-up: See this page for some of their AC-DC converter products. See their Compliance page for more information on efficiency ratings.

Note: Some people use the term "wall-wart" for the transformer-based devices only, while others use it for both switch-mode and transformer-based wall plug-in AC-DC converters.

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Some very helpful answers here, but this one was the most helpful to me with the pointers about switch-mode PSUs. Thanks! –  Kaelin Colclasure Nov 3 '10 at 18:04
    
@Kaelin - I'm glad that it was helpful, but you can feel free to wait a while for even more helpful answers to filter in. 24-48 hours (probably more while we're small) is a good reference. You can click the green arrow again to remove/postpone your answer selection. –  Kevin Vermeer Nov 3 '10 at 20:05
    
If it gives you a message about "You can't revoke your choice until this answer has been edited", just add another comment and I (or another user with edit privileges) will do a touch or trivial edit to give you that ability for a few hours. –  Kevin Vermeer Nov 3 '10 at 20:06
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A billion percent? I would think it stops precisely at 100. –  Kaz Apr 12 '13 at 3:14
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How much gas is wasted by a car? It depends on the manufacturer and particular model.

Old AC/DC converters just had a large transformer, some diodes, and a capacitor, but nowadays many are switching converters that offer better regulation, improved efficiency, and smaller size.

From the couple transformers I've looked at on Digi-Key, all the Energy Star level IV and V transformers have a maximum of 0.5 W usage at no-load.

Energy Star has a whole report on this, and fancy graph (on page 5):

alt text

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This is the best answer. Shoulda gotta tick. –  Rocketmagnet May 5 '12 at 20:47
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Numbers:

The best ones will waste < 0.5 W when no load is connected, but the no-load-losses can be as high as > 10 W, depending on the design, and even for switch-mode wall warts.

My guess is that most typical wall warts used today consume about 1...3 W when no load is connected.

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Do you have a source for those numbers? I know that CEC Compliance requires a <0.5W no-load dissipation, but I'm not sure about your guess. –  Kevin Vermeer Nov 3 '10 at 15:29
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"Hand test": Most wall warts feel like they dissipate about 1...3 W. This is only a guess, though, I admit, but it seems reasonable: The bulk voltage at the primary side is often around 320 V. The typical supply voltage for the primary control circuit is around 12 V. In no-load, the aux winding will not provide enough energy to keep the primary side alive, so a resistor will have to drop ca. 300 V at ca. 2...5 mA, which is the typical current that a cheap design needs at the primary. 300 V * 4 mA = 1,2 W. The no-load efficiency of most cheap switchers is not good except for some newer designs. –  zebonaut Nov 3 '10 at 15:35
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"Hand test" - I like it. –  Kevin Vermeer Nov 3 '10 at 15:48
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Not enough to worry about. If a bad one wastes 10 W (according to zebonaut), that will be a total energy waste of 7 kWh per month. This actual test of real wall warts measured 2 W max (1.5 kWh/month)

By comparison:

  • A single hour of running an electric oven uses 2 kWh
  • A single load of laundry in a hot clothes washer uses 4.5 kWh
  • A single load in an electric dryer uses 3.2 kWh

If the wall wart is warm to the touch when unloaded, it's wasting energy and you could try to unplug it when not in use. If it's cool to the touch, then don't worry about it.

SMPS supplies and AC supplies shouldn't draw too much (an unloaded transformer is just an inductor, and only the residual resistance is wasting energy). DC supplies with linear regulators inside would be the worst.

In general, don't worry about it. It's straining out gnats while swallowing camels. :D

And in winter, definitely don't worry about it. The "wasted" energy is heating your home and making your heater work less. All household appliances magically become 100% efficient in the winter.

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Gas/LP/oil is about half the cost of electricity per BTU of useful heat you get from them. –  Nick T Nov 3 '10 at 17:00
    
Half the cost of an inconsequential amount is still inconsequential :) –  endolith Nov 3 '10 at 18:45
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If you use a heat pump instead of wall warts you get 500% efficiency from electricity to heat during winter. 100% is not something to strive for when using electricity to get heat. –  Macke Nov 3 '10 at 22:27
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Depends on the wall wart.

For example some cheaper ones may use higher leakage capacitors which might waste a tiny bit of current. And the diodes might drop some voltage and waste this as heat when charging up this leakage. And then you've got other losses, like those of the transformer, or in some cases of the surge protection devices (MOV's) across the input.

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