Lawrence Finch wrote an article at Everything you wanted to know about charging your iPhone or iPad . Theoretically this is a great article and I know many people in internet thank him for his explanation. Unfortunately he missed out something - the facts!
Finch said:
If it takes your iPad charger (2.1A) 5 hours to charge up your iPad with the iPhone charger (1.0A) it will take 10 hours to charge it because it is about 1/2 the watts and amps.
I did a lab test using a current meter and it showed that Finch was wrong about the statement above.
When you charge an iPad Air with an iPad charger (2.1A), the current flowing into the iPad is NOT 2.1A. You will get not more than 1.6A when your iPad's battery is almost flat. But Finch assumed that the current is 2.1A. The iPad charger is just a transformer and techinically it shouldn't be called a charger. The actual charger is residing in your iPhone/iPad and it is the one that is regulating the power. I do not know why, but the iPad is only allowing less than 1.6A entering it (and not 2.1A).