Charging an iPhone, difference between USB port and power outlet

Solution 1:

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.

Solution 2:

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).