It is widely known that most AC-DC converters (aka "wall warts") continually use electricity, even when no devices are plugged in to them. That wasted energy is often called "phantom electricity" or "phantom use".

wall wart

A new trend is USB (DC) outlets, whether integrated into the wall plate:

wall plate USB

or attached to an existing AC receptacle:

add-on USB

Do both of these two new types of AC-DC converters waste energy just like the old wall warts?

If so, what is a reasonable solution?


4 Answers 4


Electronically they're the same and will waste as much energy as a conventional adaptor. Depending on your electrical regulations they may be forced to be at the good end of adaptor efficiency, or they may be at the very cheap end. This will be a matter of safety regulations, but the cheapest designs fall down on safety and efficiency.

I hunted for one with a switch on the charger and failed to find it. But there are built in chargers without mains sockets, which could be connected via a switch.

  • Thanks. Currently, I use wall warts connected to switches (to use less energy), but it is quite a mess. Also, I incur the cost of those switches, and the planet incurs the environmental and energy costs of producing those switches. Commented Dec 8, 2015 at 8:29
  • @RockPaperLizard that's a good point. The marginal materials cost of switched sockets at installation time will be tiny, I would guess that for typical efficiencies of modern switch-mode power supplies at idle you might use fewer resources not fitting the switch, especially if you unplug for long periods at idle and use good quality power supplies. Without the ability to unplug the built-in adaptors remove this option (the only one we have is on the same circuit as the fridge).
    – Chris H
    Commented Dec 8, 2015 at 9:28
  • We use what's effectively a powered USB hub so we can charge multiple devices from a single power supply, as well as preferentially using the USB outlets on the computer for charging since the computer has to produce 5V anyway.
    – Ⴖuі
    Commented Dec 11, 2015 at 21:24
  • 1
    @Ⴖuі, that might well help but given how variable efficiency can be both as a function of load and between makes/models it would take a lot of work to be sure what's most efficient. It wouldn't even translate well from one person's hardware to another's.
    – Chris H
    Commented Dec 11, 2015 at 21:51

If you are looking for an efficient USB outlet, this post refers to a USB wall outlet that shuts off when not plugged in.

  • That is a fantastic design and implementation! The cost, however, is astronomically high. Financially speaking, it might take decades for such a device to pay for itself. Environmentally speaking, it might be a net gain both for new installations and where the existing receptacle could be reused in another location. Commented Dec 10, 2015 at 9:26
  • On thinking about it more, it's not really a fantastic design and implementation. When a USB cord is plugged into the outlet, it will still draw power, even when no device is connected to the USB cord. Since outlets are commonly located behind desks, it would be normal to have one or more USB cord(s) plugged in at all times, even when no devices are attached. Commented Dec 11, 2015 at 22:14
  • 1
    You've got a good point. It does seem like a good design on a first look but practically it would end up being worse due to the high initial cost. If you think about it, there can't be a perfect solution for this problem. There will always be a small power leak. I wonder if there is a energy certification for these sockets.
    – Vinod
    Commented Dec 12, 2015 at 4:52

I was looking for similar information and came across this article: "Tested: Should You Unplug Chargers When You’re Not Using Them?".

Seems that, while some of the wall warts may draw more power, modern ones use almost nothing. Probably not worth buying anything fancy to address the "problem".


Install a GFCI outlet on the same circuit as your USB integrated outlet. When not in use, just trip the GFI outlet and kill the whole circuit. Might not work in all cases as a lot might be tied to the same circuit, but this works in my house.

  • Thanks Bob. That's a clever trick. How dirty is the power when you cut it and restore it via GFI? I'm wondering if you get spikes on that circuit or others nearby. Commented May 9, 2020 at 6:58

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