Obviously, I am talking about simple charger without any indicator lights etc. If yes, why does it use energy when the circuit is not closed?

Examples of the claim:

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    see: superuser.com/questions/70334/…
    – vartec
    Commented Dec 19, 2011 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
    Commented Dec 24, 2011 at 19:41
  • 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. Commented Oct 14, 2013 at 0:03

3 Answers 3


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
    Commented Jul 3, 2012 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
    Commented Apr 3, 2013 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
    Commented Sep 9, 2013 at 13:50
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    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. Commented Jan 2, 2014 at 19:52
  • @mcv To make a charger "interrupt" the circuit, it needs to be powered. For that you need something like a charger, which... you get the drift ;-)
    – hdhondt
    Commented May 18, 2016 at 6:41

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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    Usually a home will have a master electric switch somewhere as part of safety equipment such as in a fuse box. So why not flip it off at night or when away and turn it on when needed? Hint: <blink>12:00</blink>
    – Paul
    Commented Nov 17, 2014 at 12:17
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    @Paul because it also controls heating, hot water and light?
    – mmmmmm
    Commented Nov 18, 2014 at 15:12
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    @slebetman: Do all sockets have their own switches where you are? In the US they do not. Commented Jan 16, 2015 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
    Commented Jan 16, 2015 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
    Commented Feb 24, 2015 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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