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see: superuser.com/questions/70334/… –  vartec Dec 19 '11 at 15:38
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Any electronic device with a transformer in it (such as a cell phone charger) can consume energy while powered off, because the transformer completes an electric circuit. The only way to completely eliminate power consumption is to break the electric circuit before the transformer. –  Flimzy Dec 24 '11 at 19:41
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There is clearly a question worth asking as to whether green groups who want us to feel guilty for not turning everything off are making a symbolic or meaningful point. –  matt_black Mar 4 '12 at 20:23
    
Anecdotal evidence from 3 chargers of an Siemens, Motorola and Samsung cell phone, while charging at 4, 3, and 3 Watt the measuring tool (which is made for higher voltages and not very accurat for low values) always reports 0 watt if no phone is connected. The maximum precision for the tool is 1 watt. –  user unknown Oct 14 '13 at 0:03
    
I don't think it's that green groups want us to feel guilty for not turning everything off, it's the opposite. They want us to feel good for turning something off, and for us to associate that good feeling with them, so they exaggerate the effectiveness of turning off a charger. –  bdsl Feb 24 at 0:10

3 Answers 3

up vote 48 down vote accepted

Inside virtually every phone charger is a transformer. Transformers have a finite resistance, and hence there will always be current flowing through them if they are plugged in, even if there is no load (i.e. nothing charging). That's basic physics.

But the obvious follow-up question is: how much energy does it use?

Estimates vary, but its certainly not much. This article claims (without reference) that its 1-5 watts. This page claims that it's less than half of a watt. And further claims that this represents one hundredth of a percent of a typical person's usage. This article gives more figures for different standbys, and gives that for a phone charger at about 0.4W. You can consume that power for 380 days to get the same amount of energy as a hot bath. Of course with very many people using phone chargers, that can add up to a very large amount of electricity.

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WRT to the last sentence, yes, but... if everyone does a little, we'll achieve only a little. –  Benjol Jul 3 '12 at 11:36
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The discrepancy between the reports may well be caused by differences between makes and models of chargers. –  jwenting Apr 3 '13 at 6:17
    
0.4W for a limited period is nothing, but 0.4W day in day out adds up. Still not to a huge amount, but more than is necessary. I too should pull my charger out of the wall more often. Although I wonder how hard it is to make chargers actually interrupt the circuit while not used. –  mcv Sep 9 '13 at 13:50
    
A lot of us have a variety of chargers, for different devices or simply in multiple locations for the sake of convenience, so per household it may add up more than you'd at first think by considering a single charger. –  Larry Gritz Jan 2 '14 at 19:52

In this Green Monk article, it is shown (with an ammeter) that most chargers idle between 0 and 0.1 watts.

The mobile phone chargers I tested all consumed 0.1W or less of electricity when left plugged in and not charging a phone. That is minute.

It contrasts that with other devices.

Well, my microwave consumes 3.5W when plugged in and not in use (that’s 35 mobile phone chargers worth), my printer draws 5.9W when on and not actually printing (59 mobile phone chargers worth), my Nintendo Wii draws a whopping 9.5W when on and not in use (95 mobile phone chargers worth), even cradles for cordless home phones can be consuming eight times more electricity than mobile phone chargers!

It concludes that you should unplug all devices (including microwaves, printers and games consoles) when not in use, but that chargers are a low priority, in comparison.

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It's a good idea to have everything on powerstrips that are easy to turn off. No messing with plugging and unplugging cords, just one button, and your Wii, printer, etc become usable. It gets harder for microwaves that are builtin, though. –  mcv Sep 9 '13 at 13:54
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@Paul because it also controls heating, hot water and light? –  Mark Nov 18 '14 at 15:12
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@slebetman: Do all sockets have their own switches where you are? In the US they do not. –  Nate Eldredge Jan 16 at 4:41
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@NateEldredge: Yes they do which is why it didn't make sense to me to unplug: upload.wikimedia.org/wikipedia/commons/c/c4/… –  slebetman Jan 16 at 7:18
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@slebetman socket switches are not used in most countries (I have seen them in UK, Australia and New Zealand, definitely not in US and mainland Europe). –  nico Feb 24 at 0:21

This question reminded me of the xkcd what-if post which mentions that "If an unused charger isn’t warm to the touch, it’s using less than a penny of electricity a day", but without references. Searching more I found the reddit thread following the xkcd post, which has lots of links, including to pages 68 and on of the free (Creative Commons) book "Sustainable Energy—without the hot air"[1], which was already linked to in a comment by Benjol on this thread.

Page 72 of the book mentions the study Calorimetry for power conversion in mobile telephone chargers[2] conducted in the University of Cambridge used calorimetry and found power consumption of 0.472W, and thus approximately 0.011kWh per day in a popular Nokia charger, and similar results for other chargers. A follow-up study was performed a few years later "Precision Calorimetry for the Accurate Measurement of Losses in Power Electronic Devices"[3] that validated the methods used in the original study, and actually cites "Sustainable Energy—Without the Hot Air" as a reference.

[1] MacKay, David JC. "Sustainable Energy—Without the Hot Air, 2008-Dec 2. Cambridge (UK): UIT."

[2] Weier, S., and R. A. McMahon. "Calorimetry for power conversion in mobile telephone chargers." In Universities Power Engineering Conference, 2007. UPEC 2007. 42nd International, pp. 986-991. IEEE, 2007.

[3] Weier, Sven, Mohsin A. Shafi, and Richard McMahon. "Precision calorimetry for the accurate measurement of losses in power electronic devices." Industry Applications, IEEE Transactions on 46, no. 1 (2010): 278-284.

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