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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
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2 Answers

up vote 33 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 at 19:52
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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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