你的电脑在等待你的命令的时候会消耗大量的电能，那么用USB接口给智能**或平板电脑充电会对它产生很大的需求吗？

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

可在WallperSwide以墙纸形式提供图像。

问题

超级用户读者阿内赫很好奇，如果给USB设备充电会给他的电脑带来额外的负荷：

Something I’ve always wondered. If I c***tantly hook up cellphones, hard drives and the like via USB to my computer, will it eat up more on the electricity bill? Or are the USB ports using up electricity by just being enabled anyway, thus not affecting the power usage?

那是怎么回事？当你通过电脑给**充电时，会消耗多少能量？

答案

超级用户撰稿人Zakinster深入了解了从电脑充电与从墙上充电器充电的区别，以及两者的效率：

Short answer: Probably, but not necessarily; it won’t be free power, but it might be obtained more efficiently. It really depends on the particular power supply’s efficiency curve, and the point at which you are operating it at (and power c***umption is affected by software).

Long answer:

A USB port can output maximums of 500mA (USB1&2) and 950mA (USB3) at 5V which gives maximums of 2.5W (USB1&2) and 4.75W (USB3).

USB ports don’t c***ume power by themselves. Without anything plugged, they are just open-circuits.

Now, if you get 1A (5W) out a USB3 port, it will usually increase the global power c***umption by ~6W (depending on your power supply efficiency) which would be an increase of 2% to 5% of your computer power c***umption.

But, in some cases, it may be different.

If you take a look at some PSU efficiency curve (from AnandTech) :

You’ll see the efficiency is not a c***tant value, it varies a lot depending on the load applied to the PSU. You’ll see on that 900W PSU that at low power (50W to 200W), the curve is so steep that an increase in the load will entail a substantial increase in efficiency.

If the increase in efficiency is high enough, it would mean that in some cases, your computer may not need to actually draw an extra 5W from the wall socket when you’re drawing an extra 5W from a USB port.

Let’s take an example of a computer drawing 200W on a PSU with an actual efficiency of 80% at 200W :

Computer power c***umption : 200W USB device power c***umption : 5W PSU efficiency at 200W : 80.0% Wall power c***umption without USB : 200W / 80,0% = 250.00W

Now, depending on the efficiency curve of the PSU between 200W and 205W, the relative power c***umption of the USB device may be completely different :

<Case 1> PSU efficiency at 205W : 80.0% Wall power c***umption with USB : 205W / 80.0% = 256,25W Wall power c***umption of the USB device : 6.25W

This is the usual simplified case, where the efficiency is the same, hence the power c***umption of the USB device is equivalent to 5W / 80.0% = 6.25W

<Case 2> PSU efficiency at 205W : 80,5% Wall power c***umption with USB : 205W / 80,5% = 254,66W Wall power c***umption of the USB device : 4.66W

In this case, the PSU efficiency is increasing between 200W and 205W, thus you can’t deduce the relative power c***umption of the USB device without taking into account the whole computer power c***umption, and you’ll see the relative increase at the wall socket may actually be lower than 5W.

This behavior only happens because, in that case, the PSU is under-loaded, so it’s not the usual case, but it’s still a practical possibility.

<Case 3> PSU efficiency at 205W : 82% Wall power c***umption with USB : 205W / 82% = 250,00W Wall power c***umption of the USB device : 0W

In this case, the PSU draws the same power from the wall socket, whatever the load it receives. This is the behavior of a zener regulator where all unnecessary power are dissipated into heat. It’s a behavior that can be observed in some kind of low-end PSU at very **all load.

<Case 4> PSU efficiency at 205W : 84% Wall power c***umption with USB : 205W / 84% = 244,00W Wall power c***umption of the USB device : -6W

That last case, is a purely hypothetical case where the PSU would actually c***ume less power at higher load. As @Marcks Thomas said, this is not something you can observe from a practical power-supply, but it’s still theoretically possible and proves that the instinctive TANSTAAFL rule cannot always be applied that easily.

换言之：如果你绝对关心效率，那么使用你的电脑为尽可能多的USB设备充电（当你在那里使用电脑时），而不是为每个设备**一个独特的墙疣。但实际上，损失和收益是最小的，您应该以最方便的方式为设备充电。

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