The power outlet has more power obviously, but maybe the computer USB is suffice to charge it at maximum speed.
Is there a difference between charging an iPhone using the computer USB port and using the power outlet?
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Here is a page where you have the result of the comparison of charging an iPhone4 from USB or power outlet.
Here is their conclusion:
Stating the obvious, the iPhone 4 takes longer to get a full charge on USB than an outlet. What you may not have realized is how big the difference was, an extra 30 minutes! That’s 23% longer to get a full charge on the USB.
The standard USB spec is 500 mA maximum current. The charger provided with your iPhone can provide up to 1000 mA.
So in general, I would expect it to take about twice as long on a typical computer.
However... many newer Apple computers can actually provide up to 1100 mA USB power (according to this Apple Knowledge Base article) for a limited number (1 to 3) of devices plugged directly into the computer (i.e. not via a USB hub). The iPhone will supposedly draw 1 A when connected in this way (according to the System Profiler at least), so as far as the published specifications go, it shouldn't make any difference.
Some opinion/speculation: It seems strange that the page linked to by LudoMC claims a significantly longer charge time from a laptop that's listed among those capable of delivering 1100 mA to a single device. It makes me wonder if Apple's laptops are really meeting the 1100 mA spec they claim, or whether there was another device connected to the laptop in the test (they don't mention any other devices, or the lack of them, nor do they mention the greater-than-standard power capacity of their MacBook, so they may not have been aware of it and/or the fact that only 1 device can utilise it).
Officially, no. In practice, the chargers that plug into a power outlet can deliver more current than what is mentioned in the USB specification, so are noticeably faster at charging many devices.
In addition, when not plugged into the computer, a device is usually off more of the time, and devices that are off charge faster.
When I charge my mobile through power plug socket, it takes just one (1) hour to become full charge i.e. from 1-2% to 100% as well as through CPU charging, it takes one hour fifteen minutes to become full charge. But, battery lasts long in CPU charging, because drainage of battery charging is slowly reduced. Whereas in power socket charging, drainage of battery charging is quickly reduced. Therefore, I suggest all of you that though charging time is much longer via CPU charging than power socket charging, yet it will be better to charge a mobile via CPU charging so far as drainage of battery is concerned.
Other answers are good, but an important thing has not been explicitly mentioned is that you can actually use a PC to charge iOS device quickly. This depends on whether your system has available drivers.
For example, on Linux systems since kernel version 5.7 there's a driver shipped which adds fast charge. Quoting the description, as it may be interesting in this thread:
iOS devices will not draw more than 500mA unless instructed to do so. Setting the charge type power supply property to "fast" tells the device to start drawing more power, using the same procedure that official "MFi" chargers would.
Linux systems aside, I think Apple computers can do that too.