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I work with a computer and I was wondering if using its USB ports to charge the devices that I use the most (mostly my phone and my e-reader) while working as opposed to charging them using a normal wall charger would make any impact.

I am interested in two things. First, would I save any money over the year? (We can assume that I charge my phone daily, my ebook once a week and some other devices once in a while)

Secondly, does this make any environmental impact? Am I actually decreasing the quantity of electricity that I use or just making the computer to use more energy to compensate the charging of the devices?

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To give a definite answer would require a lot of research and it would probably be quite impossible to give a "for sure" answer because there are some variables. Some computer power supplies are much more efficient than others and some wall chargers are much more efficient than others.

This being said, it is generally MUCH more efficient to use the computer USB (while you are using the computer). This is because the computer is already supplying the 5V power so you are just drawing a little more off an existing load rather than powering a whole new power supply (the wall charger).

Remember, when I say much more, I'm talking in terms of percentage. The fact is that the amount of power used to charge the average phone or tablet for a year is pretty minimal and you will definitely not notice a difference on your power bill if you charge it one way or the other (as long as you unplug your computer and/or your wall charger when not needed).

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Computer PSUs generally only hit their rated efficiency when well loaded. Most desktop PCs have larger PSUs than they need, to allow for expansion, and so they are not fully loaded. Consequently it is quite possible (though by no means guaranteed) that charging from a desktop computer is more efficient than doing so from a dedicated adapter, since part of the extra load might be absorbed by increased efficiency. This is unlikely to apply to laptops.

Perhaps of at least as much import is that most computer USB sockets only deliver 0.5A, and hence will charge your phone slower than a dedicated charger. Since charging a battery faster usually generates more heat, the slower charging may again result in less energy loss (as well as perhaps a slightly longer battery life).

The difference from either of these effects will be very small, though. My general advice would be "don't worry about it". In case it isn't obvious, never leave a computer running for the sole purpose of charging a USB device - this would be very inefficient.

  • The larger power supplies aren't just for expansion, but for varying loads. As you can see if you have a load monitor running on your system. For instance, just sitting here browsing StackExchange, I'm running at 2% of CPU capacity, with fans off. Doing some intensive number crunching will get all CPU cores to 100% & spin up the fans. If I do it on the desktop, it might well load up both power-hungry GPUs and their fans... – jamesqf Feb 26 '16 at 19:27

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