While Apple’s pack-in chargers for the iPhone and iPad provide just the right amount of power for each device, charging iOS devices on desktops and laptops can be more of a crapshoot. Luckily, it’s not too hard to take control of the situation and ensure that your device is getting the juice it needs. If you’re having problems charging or upgrading your device on a PC, you’ll want to make sure it’s plugged in to a port on the machine—not a hub—and try to unplug any other unnecessary USB devices while you’re at it. Perhaps unsurprisingly, users of Apple’s Mac computers have things a little easier.
On Macs newer than 2007, all your USB ports should supply at least 1100 mA of charging power to any device—such as an iPhone or iPad—that needs it. The extra power is doled out on a first come, first served basis, so the first device you plug in is virtually guaranteed to receive the maximum amount of power, while later devices are less likely. If you have a Mac that was built this year, it might even support full 2.1 Amp fast charging of the iPad. Mac users also have a very simple way to check how much juice each device is receiving. Simply go the the Apple menu, select About This Mac, and hit the More Info… button, which launches System Profiler. From there you can select USB from the sidebar under Hardware, and select the device you want to check on. It’ll show you the Current Available, the Current Required, and the Extra Operating Current—by adding the Available and Extra Current together, you’ll get the current power output for that port. For more info, see this Apple Support document.