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Do DC adapters consume energy when no device is drawing DC current?

Or… Can I leave my mobile (5-20 W) or laptop (40-60 W) chargers plugged in and switched on and hope they won’t draw current if my mobile or laptop isn’t plugged in to the charger?

The confusion arises because if we were only considering pure DC or pure AC then we know that as long as circuit isn’t complete, no power is consumed. But when there’s a AC to DC adapter, I don’t know what's going on in that box (maybe the converter is consuming power even though nothing is plugged in.

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    As a very rough rule of thumb, I always compare the temperature of the charger when it is supplying power against when it is not supplying power. I have always found them too be much warmer when power is being consumed.
    – Lefty
    Dec 16, 2020 at 18:41
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    The Electrical Engineering community is a better place for this question. They can probably tell you about linear regulators, SMPS, 80 PLUS PSUs, idle power draw, and more.
    – Nayuki
    Dec 17, 2020 at 4:02
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    Technically speaking, you don't even need a closed DC circuit to consume energy. Batteries lose power all the time too. Good adapters leak even less than that; poor ones can go around a watt when idle - but even that's a tiny amount of power. Unless you're going really hard avoiding electricity waste, it's nothing to worry about.
    – Luaan
    Dec 17, 2020 at 12:45
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    @RockPaperLz-MaskitorCasket That same argument works for every little tiny bit of waste we make. That doesn't mean it's worth the effort to remedy that particular tiny bit of waste. Just think about all the water wasted whenever you wash your hands or flush your toilet.
    – Luaan
    Dec 17, 2020 at 20:00
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    I find it hard to imagine a world in which every person on the planet has, on average, connected at least 100 AC/DC adapters at any one time. (which is how many would be required to make the number "likely trillions" - that estimate is at least an order of magnitude too large). @RockPaperLz-MaskitorCasket
    – Phil
    Dec 17, 2020 at 22:19

6 Answers 6

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This really depends on the inner workings of the individual adapter. A cheap one might just continue running, while a more elaborate one will turn itself (almost completely) off.

For example, Nintendo's power adapters for the Wii U and 3DS are basically dormant when not connected (the current is simply too low for measurement; at least in my case).

If you're curious, you can buy an energy meter, which is like a plug you put between the outlet and the device you want to measure. It will tell you the current amount of power consumed as well as the consumption over time.

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    trust Mario to use Nintendo as an example :)
    – PeterH
    Dec 16, 2020 at 16:49
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    Does the energy meter switch itself off if the adapter switches itself off though?
    – rghome
    Dec 17, 2020 at 8:14
  • The "elaborate" ones, which have negligible standby power loss, are called switched-mode power supplies.
    – 200_success
    Dec 18, 2020 at 4:58
  • @rghome No, it shouldn't. But the energy required by the meter itself isn't counted by the meter, only what's taken through the socket on the meter.
    – Mast
    Dec 18, 2020 at 5:11
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    You can tell if you have one of the old cheap ones by just feeling it. It'll be warm.
    – candied_orange
    Dec 18, 2020 at 10:51
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All power adapters have some parasitic loss inside them such as:

  • switching loss from hysteresis of the transformer
  • switching loss from transistors (leakage current)
  • Partial conduction from movs/x/y caps (age/wear)

However like Mario has advised, they do ramp up/down subject to demand, but only for smart chargers like your laptop and not so much for basic devices such as wall warts.

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    When I looked into this issue about a decade ago, about 98% of chargers I was using had measurable parasitic loss. I now unplug stuff I'm not using.
    – RockPaperLz- Mask it or Casket
    Dec 16, 2020 at 8:58
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    "Measurable" is not the same as "significant".
    – Ian Kemp
    Dec 17, 2020 at 14:09
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    @IanKemp Multiply by billions of people with these power adapters, and it quickly becomes very significant. It's similar to recycling or voting: your singular action makes only a little difference, but collectively, it makes a significant difference.
    – RockPaperLz- Mask it or Casket
    Dec 17, 2020 at 17:24
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    @RockPaperLz-MaskitorCasket I'm not sure exactly what's changed; it's a slightly different area of engineering than I work in. But I suspect more of the high-end ones are either 1) using SiC MOSFETs as the switching elements, reducing switching losses (SiC devices have only become available for reasonable prices in the last five or so years) and/or 2) having some kind of controller that detects when there's no load and turns off the (mains-side) switching element (i've seen this even in cheap ones, sometimes annoyingly; it can turn off when there's a very light load that it thinks is no load)
    – Hearth
    Dec 17, 2020 at 17:36
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    @RockPaperLz-MaskitorCasket The really old ones, the ones from the 90s/early-00s and earlier that are real heavy, are very inefficient; those use a mains-frequency transformer (big, heavy, expensive, inefficient) and a simple bridge rectifier and linear regulator (terribly inefficient), but modern ones tend to use a much more efficient configuration that relies on having good affordable switching FETs, either a forward converter or a flyback converter or some fancy topology I'm not as familiar with. These are much smaller and lighter, so you can tell them apart pretty easily.
    – Hearth
    Dec 17, 2020 at 17:47
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I hooked up a Lenovo 65-watt adapter to a good Wattmeter and it drew less than 1 watt after the first "surge" of plugging in (2 watts). I tried an Apple adapter and it did not even register.

Ordinary good quality adapters do not draw appreciable current and can be left plugged in without any concern.

I trust this helps.

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    At $0.15/kwh, 1 watt costs about $1.31/yr
    – BlueRaja - Danny Pflughoeft
    Dec 16, 2020 at 16:47
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    @BlueRaja-DannyPflughoeft, most draw far less than one watt. It's just that one watt is the minimum that most wattmeters can measure.
    – Mark
    Dec 16, 2020 at 19:04
  • $0.15/kwh is atypical, but yeah, a buck a watt a year is about right and is my rule of thumb. Or for night lighting, half that.
    – Harper - Reinstate Monica
    Dec 17, 2020 at 23:26
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    @BlueRaja: Yes, bitcoin mining is probably the bigger problem in terms of unnecessary waste.
    – Peter Mortensen
    Dec 18, 2020 at 13:52
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All of the answers here have been useful in parts to answer my question, so I'm creating a community wiki answer. Please scroll and upvote them!

It will depend on the quality of your adapter. So unless you're sure, better turn it off.

Mario's answer gives us a way of testing how your adapter behaves, using which John's answer reported how two laptop adapters behave, which can be indicative of other brand adapters as well. Another way to find an upper limit for power drawn while nothing is plugged in is to see if your adapter conforms to some standards. user2813274's answer contains some information about one of these standards -- U.S. Energy Star spec.

binaryOps20's answer details the reasons an adapter may draw power even when no device is plugged in.

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  • Oops, looks like it'll be 2 days before I can mark this as accepted.
    – Peeyush Kushwaha
    Dec 16, 2020 at 13:18
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    It's been only 6 hours since you've asked the question - consider waiting a bit longer, someone may post a comprehensive answer which would, ideally, give specific numbers or reference good sources.
    – gronostaj
    Dec 16, 2020 at 13:36
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    @PeeyushKushwaha Thank you for truly being a community member here despite the (relatively) low feedback. It’s great to see someone truly engage with this community as a community!
    – Giacomo1968
    Dec 16, 2020 at 20:52
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I'm not finding the UL specifications, but the U.S. Energy Star specifications require less than 0.5 W for up to 10 W chargers and less than 0.75 W for 10-250 W chargers.

I forget where, but I think 80 plus or similar has requirements for idle desktop power supply usage (with a small amount more allowed for WOL).

I am not finding it, but Energy Star again has a rating of less than 2 watts (or some complicated equivalent).

The thing that I've noticed really draws power is the cable modem for TV's. Unplugging that is the same as all the phone chargers + laptop chargers combined.

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  • Yeah, those cable boxes are just computers, and when you turn them "off" you just turn off the video output. If you've ever had to actually reboot one you can see how painfully slow they are so if it actually shut down when you pressed power off, you'd be waiting a while after turning it back on to watch anything. As a result they are just always running, using close to full power.
    – briantist
    Dec 17, 2020 at 1:12
  • I don't think 80+ has any standby requirements for desktop PSUs. I've never seen it tested by any of the sites that do load/etc testing, and since standby behavior is controlled by the CPU/mobo the PSU is hooked up to most of the key factors aren't in the PSUs control anyway. About all they can control is the amount provided, 2-3A @5V on the few I spot checked, and conversion efficiency which I remember being lousy (50-60%) when it was tested. Prior to 80+ titanium which added a 10% requirement the minimum load with efficiency reqs was 20%.
    – Dan Is Fiddling By Firelight
    Dec 17, 2020 at 2:37
  • @briantist Not "close to full power", far from it. A mobile phone is a similarly powerful computer that's also always on; forget the screen on and the battery lasts half a day, forget the phone and it'll be 1-2 weeks until it reminds you of itself due to low battery.
    – mossymountain
    Dec 17, 2020 at 7:53
  • cable boxes aren't designed to run on a battery so they have different concerns (and don't have a screen to power, sending the video signal is much smaller than lighting and powering a display). But their design has probably changed too, I must admit the last time I've measured (or even had) a cable box was probably nearly or over a decade ago; at that time there was no noticeable difference in power usage.
    – briantist
    Dec 17, 2020 at 16:14
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Yes, don't be fooled... they consume a "quiescent current" which although very low, it is a value that can be measured. This is to power up the sensing circuitry so they know that something is or isn't plugged. It is usually negligible but add many of them and things start to escalate... not a lot, but still something if you are really worried about that extra couple dollars on your bill. Once you plug in a device, then the power consumption will drastically increase to supply the required charge.

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