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Voltage Adapters Power Use

  1. Aug 18, 2015 #1

    I was just wondering, does all the power get wasted in adapters? ( I mean phone chargers etc.)

    I mean, if I plug my adapter in the 240 V wall outlet which has a specific amperage and charge my phone, will my electricity bill say that I used the power resulting from the the 240V*(the current) [From the wall outlet] or will it say much less ( the power required by the phone, which is let's say 5V*the current).

    Thank you for your answers!
  2. jcsd
  3. Aug 18, 2015 #2


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    Staff: Mentor

    The power company measures the incoming volts and amps, but if the charger is reasonably efficient, the difference shouldn't be large. A typical charger runs at perhaps 20 Watts input and 10 Watts output. I recently acquired a means to test this, so if I remember tonight, I will.
  4. Aug 18, 2015 #3


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    Gold Member

    Your adpapter contains some kind of transformer, maybe a small toroid transformer working at 100 kHz. The output from the transformer is rectified into a DC voltage/current. Say the adapter has an efficiency = 90% and the phone battery has an efficiency = 90% as well, then you will have to pay 100% / 0.92 = 123% of the power used by the phone.
  5. Aug 18, 2015 #4


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    If you leave the adapter plugged in permanently, it will consume a few watts (you can usually feel it stays pretty warm). Russ will measure that too, I hope....
  6. Aug 18, 2015 #5


    Staff: Mentor

    Wikipedia AC adapter says.

    External supplies are usually left plugged in even when not in use, and consume from a few watts to 35 watts of power in that state. The report concluded that about 32 billion kilowatt-hours (kWh) per year, about 1% of total electrical energy consumption, could be saved in the United States by replacing all linear power supplies (average efficiency 40–50%) with advanced switching designs (efficiency 80–90%), by replacing older switching supplies (efficiencies of less than 70%) with advanced designs (efficiency of at least 80%), and by reducing standby consumption of supplies to not more than 1 watt.[8]

    That is HUGE. The greens should fight to unplug all those wall warts and all devices running on standby or sleep mode waiting for the remote, or for someone to touch the keyboard, to turn it on. It is hard to imagine a more wasteful use of electricity.
  7. Aug 18, 2015 #6
    Thank you Hesch! :) Although, I still have some problems understanding what does electricity consumption mean...

    In the adapter you basically have 2 coils of wire, through the first your wall outlet's power passes through right? So why doesn't that count as "use of electricity", why isn't that measured by your watt-hour meter too?

    I understand that in the second coil of wire your desired power for your let's say phone gets induced, so the power which is induced into that will appear on the watt-hour meter.. but why is that?
  8. Aug 18, 2015 #7


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    As I said, the power through the first coil is the only thing the utility measures.
  9. Aug 18, 2015 #8


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    Okeey, so let's see how it looks. FYI, the voltage was measured at 120.7V. First-up, an unconnected phone/tablet charger (2.1A rated):

    Surprising: No load detected. Next, charging a cell phone:

    So, let's math that: 5.26V at 1.17A is 6.15W or 82% efficiency. Not bad at all. Next, a tablet:

    Efficiency this time: 81% at a much higher power. So from half load and up, the efficiency is completely flat. I'll be interested in seeing if I can tell what it bottoms-out at with little load. To do that, I'll have to fully charge my cell phone and see what it does while just maintaining. Maybe later after it charges...

    Next, my laptop PSU with no laptop:

    That surprises me a lot. I was sure it would show something. Looks like these "wall warts" are actually pretty good.

    One more, for extension, the microwave:

    More math: my electricity costs about $0.17 / kWh. So per year, this microwave charges me $4.77 per year to tell me what time it is.
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