Here is how to charge your iPhone from your MacBook Air battery. (It will work for a MacBook Pro as well.)
I was recently on a car trip and my iPhone went flat. I didn’t have a 12 V adapter to plug it into the cigarette lighter in the car.
I plugged the iPhone into the USB port on my laptop, but of course it didn’t charge because the laptop was asleep. I opened the laptop, and the laptop woke up, and the phone began to charge, but this was no good because it would have drained the battery of the laptop too quickly.
So I closed the laptop, putting it to sleep, and the phone continued to charge!
I estimate that you can fully charge your iPhone 3 to 4 times from a full MacBook Air battery.