I was trying to figure out how much power I actually use when I am charging my phone. Here are the specifications of my charger:

  • Input: 100/240 V, 50–60 Hz, 0.15 A
  • Output: 5 V, 0.7 A

I've heard that in order to calculate this, you need to pay attention to the input.

$$P = V\cdot A = 240\ \mathrm V\cdot 0.15\ \mathrm A = 36\ \mathrm W$$

I charge my phone for 4 hours a day at most. In that case, I use 144 watt-hours a day, and 51840 Wh a year, or 51.84 kWh. And that seems awfully lot, considering that all the articles I've read about the power consumption of chargers stated that they used about 2 kWh a year. I know I charge my phone too much (I really need a new battery), but it still doesn't add up. Are my calculations wrong? And if they are, what is the correct number?

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    \$\begingroup\$ The input numbers are the maximum. So 0.15A would be at ~100V, and less than half of that at 240V (ballpark). roughly, if the output is 5V ~0.7A, it outputs 3.5W. If you assume 50% efficiency, that's 7W on the input. At 100V, that's 0.07A and 0.03A at 240V. Added to that, your phone does not draw 0.7A at 5V all the time that its charging, so in practice the power draw is a lot lower. \$\endgroup\$ – Wesley Lee Apr 7 '18 at 20:07
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    \$\begingroup\$ @WesleyLee You should expand that into an answer. \$\endgroup\$ – Russell Borogove Apr 7 '18 at 20:07
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    \$\begingroup\$ As a quick sanity check, any power the charger actually uses - that it doesn't pass on to the phone - would end up as heat. Put your hand on it, you might feel it slightly warm with a phone plugged in and charging; dead cool otherwise. If it really took 36W (not Wh) it would be nearly as warm as a 40W lampbulb. Pro tip : DONT put your hand on a running 40W bulb! \$\endgroup\$ – Brian Drummond Apr 7 '18 at 20:20
  • \$\begingroup\$ Yeah, that was another reason I was sceptical about my calculations (beside my almost complete lack of knowledge about this topic). I have put my hand on a 30 W bulb that has been running for hours by accident (it was not a pleasant experience, that's for sure), so I can definitely say that a charger never gets that hot, not even when it is plugged on for the whole night. \$\endgroup\$ – user184968 Apr 7 '18 at 20:48
  • \$\begingroup\$ On top of the unit error since Wh isn’t W, that rating on the nameplate is an calculated “lie” to the government/external agency which certifies it. You round the current upwards and add VAT on top of it since there is no penalty for too high whereas you can’t go over the rated current. \$\endgroup\$ – winny Apr 7 '18 at 20:53

The input numbers are a maximum or worst case scenario that the manufacturer wants you to take into consideration, they do not reflect the power draw at 100% of the time. It could be for example, some surge current when first plugging it in due to capacitors charging up, or just some huge margin.

Also, the average input current at 100V would be more than double that of when plugged at 240V.

Let's do the reverse calculations, from the output to the input:

5V and 0.7A gives 3.5W output. If you assume 50% efficiency, that's 7W on the input.

At 100V, that's 0.07A and 0.03A at 240V. (Much less than 0.15A)

Added to that, your phone does not draw 0.7A at 5V all the time that its charging, so in practice, the power draw is a lot lower.

Power supply efficiency:

This interesting article from 2012 tests a dozen chargers, from brand names to counterfeit ones, and the efficiency ranges from 60 to 80% (note: "vampire" indicates the no-load power consumption):

enter image description here

One way of guesstimating the power usage from phone charging would be:

1 - Estimate how much charge your phone holds. Let's say 2000mAh 3.7V battery, so ~ 8Wh

2 - Lets say you fully charge your phone every day.

3 - Lets assume the charger circuitry in your phone has 80% efficiency and the USB PSU 60%. So charging your phone wastes 50% of the energy.

That's 16Wh per day. ~6kWh per year. This does not take into account the power draw of the charger when your phone is not plugged in, but on the other hand I assumed pretty bad numbers for the rest of the points.

This article from 2013 by Forbes uses 5.45Wh as the battery energy, does not take power losses into account and arrives at a result of 2kWh.

Battery charging curve:

You can see in the graph below that maximum current draw falls drastically after the first hour(s) of charge. So even the numbers of 0.07A and 0.03A are the maximum for a brief time.

enter image description here

  • \$\begingroup\$ Thank you for the answer! That clears up a lot, I thought that the input numbers represented how much it draws the whole time it is being charged. \$\endgroup\$ – user184968 Apr 7 '18 at 20:37
  • \$\begingroup\$ @user184968 -- I'm glad to help! I was curious wrt the numbers myself. I just added a graph that depicts current draw during Li based battery charging, it shows how fast the current changes after the first hour or so of charging. \$\endgroup\$ – Wesley Lee Apr 7 '18 at 20:45
  • \$\begingroup\$ How come it does not need to draw all 240 V (or how much the AC current is in that country) to convert AC to DC in the transformer (which is basically what a charger does)? I'm sorry for all these dumb questions, but I don't really understand this process. \$\endgroup\$ – user184968 Apr 8 '18 at 14:52
  • \$\begingroup\$ @user184968 -- if you plug it to a 240V system, it will work at 240V, but it will draw less current on average: same power on a higher voltage draws less current. If you plug it to 120V, it will draw double the current on average. \$\endgroup\$ – Wesley Lee Apr 8 '18 at 16:59
  • \$\begingroup\$ Oh, I think I am starting to get it. So because it draws less current, the amount of power it consumes also decreases. Thank you again for explaining. \$\endgroup\$ – user184968 Apr 8 '18 at 17:16

The input rating on mains-connected devices is not there to assess power costs, but to dimension wiring (both house wiring and accessories such as power strips, plug-in timers, and extension cords). In your case, please refrain from connecting more than one hundred such chargers that are specified at a 0.15A input current to an (otherwise unloaded) circuit that is fused at 15 Ampere. The manufacturer is expected, by electrical codes of various countries, to put such a rating on any device - the only thing it is supposed to guarantee is that the device, when intact, will not exceed that input current (inrush currents likely excepted, but not discontinous use eg from a thermostat-controlled device).


Just get a Kill-A-Watt meter and check it. It probably starts out on the higher side and then ramps down in wattage. My guess is it would start out at about 10 watts and ramp down to about 5 watts, regardless of input voltage (anywhere from 100V to 240V). Kill-A Watt meters not only show real time wattage, but also line voltage coming in, power factor, frequency (such as 50 Hz - 60 Hz), input amperage.... You can also tell it how much you pay for electricity (per KWh), such as $0.13, and it will calculate how much it is costing you to charge that phone per day, week, month, year. They are really useful devices. I have 2 of them and use them a lot. Mine are for about 100-130V only. I think they also have a 200-260V version too for overseas.

  • \$\begingroup\$ This is actually a fairly good idea, not only because it shows what is actually going on in a given case (vs. guesstimates based on assumptions) but also because it shows what is going on over the course of charge. Such a meter won't be perfectly accurate, but it reveals a surprising amount of information. It's also very useful for discovering when a cable without signal wires is causing a device to charge only at 2.5 watt USB spec rate, and not at the faster rate that most charger/phone combinations support when the phone recognized it has been plugged into something better. \$\endgroup\$ – Chris Stratton Dec 15 '19 at 21:59
  • \$\begingroup\$ @Chris Stratton - to get a more accurate wattage reading, since a few watts is quite low, would be to put a known load (such as a 40 watt light bulb) as part of a combination load. Then if the charger is say 3 watts, it should read 43 watts total and you just subtract the 40 you know is from the light bulb. Also, by resetting the counter and charging over a few hours, you can get a fairly accurate average charge rate. For example, "flatten" your smartphone battery down to 0%, reset the counters on the Kill-A-Watt to 0, then check the counters when the smartphone is 100% charged. \$\endgroup\$ – David Dec 15 '19 at 22:07
  • \$\begingroup\$ Sorry I am a year and 8 months "late" answering this but hey, better late than never they say. \$\endgroup\$ – David Dec 15 '19 at 22:09
  • \$\begingroup\$ Not sure I can agree with the lightbulb idea as its consumption may not be stable, additionally it would defeat any sort of auto-ranging behavior (if a meter turned out to have that). Running a phone battery down is generally best avoided unless it's necessary to get charge tracking software unstuck. \$\endgroup\$ – Chris Stratton Dec 15 '19 at 22:12
  • \$\begingroup\$ I used to use the Kill-A-Watt meter when charging my golf cart batteries. It was fun to watch it start out at about 1000 watts (it was a 36V 25A charger), then taper down slowly. Towards the end there was a brief spike but I asked the charger manufacturer about it and they said it is normal for that charger (it was some type of proprietary charge algorithm). So you can basically approximate the charge amperage using the cheap ($20 or so) wattmeter, instead of having to buy a much more expensive DC clampmeter. \$\endgroup\$ – David Dec 15 '19 at 22:28

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