merlin1128 recently posted an excellent article about chargers for iPhones and iPads, how they work, and how to use them. His tip is reproduced (with permission) below.
I just recently got an iPad and have the iPhone 4, iPhone 4S, iTouch, Ipod, basically I have everything Apple makes. I can see that a lot of people are confused about chargers due to them showing watts, volts, & amps and since I have a very extensive background in electronics I will clarify all this in this post.
|PC USB||5 volts||0.5 amps||2.5 Watts|
|iPhone Charger||5 volts||1.0 amps||5 Watts|
|iPad Charger||5.1 volts||2.1 amps||10 watts|
Will they all charge an iPad even when your PC shows "Not Charging"? YES your PC USB will charge your iPad, but very, very slowly because it is 1/4th of what an iPad charger is so it will take 4 times longer than an iPad charger (BUT YOUR SCREEN MUST BE OFF). To test that plug your iPad into your PC USB charger before you go to bed and write down what the battery percentage is and in the morning you will see it went up. Not much, but it will go up some. Reason it says "Not Charging" when you are looking at the screen when plugged into your PC is the USB is basically powering up your screen and doing nothing for the battery.
If you use an iPhone charger it will show the iPad is charging; now look above at the numbers and you see that it is about 1/2 of what the iPad charger is so it will take twice as long to charge up your iPad using an iPhone charger. Below these are totally hypothetical numbers and not realistical and I am using them to keep it simple and understandable.
If it takes your iPad charger 5 hours to charge up your iPad with the iPhone charger it will take 10 hours to charge it because it is about 1/2 the watts and amps. Make sense? Its basically just doing the math. I have not tried to do a full charge using a USB Port on my PC, but I believe like I said doing the math it will take 4 times longer because the USB port is 1/4 the output of the iPad charger and 1/2 the output power of the iPhone charger. But remember if you have your iPad screen on the USB on your PC is just powering up your screen and doing nothing for charging the battery. To test that I would have to take a guess and say let your iPad run down to say 20% or so, turn it OFF, plug it into your USB Port and power it up after I would say at least 12 to 16 hours to see where it is at. It will charge the battery I know that for sure, but it is going to take 4 times longer.
I hope this clarifies things up for everyone on this because it is all just math with the output because they all produce 5 volts only difference is Amps and Watts and the equations for that is Volts x Amps = Watts. Apple gives you the 2.1 Amp 10 Watt charger so it won't take a day to charge your iPad due to the battery being much bigger.
Most USB Hubs produce the .5 Amps even though they have an external power supply you plug into the wall. Their are some USB Hubs that have usually 1 port that will produce the 10 watts 2.1 amps, but it will just be out of one port on the hub, so if you buy one read the specifications carefully. Car chargers are pretty much the same, so read before you purchase.
Here is the name of a USB Hub that has ONE fast charge port they call it that is the same as the iPad Charger: Symba 7 Port USB 3.0/2.0 Hub with One Fast Charging USB 2.0 Port (SD-HUB20102) if you want to charge fast and sync your iPad on your computer.
If you are looking for a Car Charger that is 2.1 amps just like the Apple Charger I got a Satechi 2412ST and what is really nice about it is that it is pretty much flush when plugged in, meaning you don't have this charger sticking out of your accessory socket it is flush which is really nice if you have a cove that needs to close you can leave this in...
I know this is long, but I hope all this helps and clears things up for everyone and if you buy any USB Hubs or Car Chargers for your iPad read carefully that it produces 2.1 Amps to get the fastest charge or if you don't mind it charging slower (1/2 as fast) you can get a 1.0 Amp one.