There's a lot of ways to recharge an iPad - thank you, USB - but the iPad is finicky. For a portable, it has high power requirements, high enough to cause problems for some people. (It even has its own support document.)
We took the four most common ways to charge an iPad: The iPad's standard 10W charger, Griffin's PowerBlock iPad charger, the iPhone's standard power adaptor and via a MacBook Pro's USB port. We timed how long it took to bring the battery up to 80 per cent, which is the standard baseline for a "quick" charge. That last 20 per cent goes very slowly to preserve battery life, so doing total time wouldn't give a fair indication of the charging method.
The standard iPad charger and Griffin's PowerBlock are both spec'd to pump out 5 volts at 2.1 amps (so ~10 watts). The iPhone's standard compact power adaptor runs at 5V at 1.1 amps (~5 watts). The MacBook Pro USB port is a little more complicated - if you have a newer Intel model, like we used, the port adapts to what you plug into it. Normally, it pumps out 5V at 500 milliamps, but if you plug in something like an iPad and it has the juice to spare, the port becomes a high-powered port, juicing your iPad at 1.1 amps, so roughly the same as the iPhone charger. (Note: All of these adapters are smart and dynamically adjust output, which is why you can plug your iPhone into an iPad charger and it won't go kablooey.)
Here's what we found after running down our iPad 3G several times - netting a damned impressive six hours and 30 minutes of battery life while continuously streaming Netflix videos over 3G - and then charging it back up to the 80 per cent mark. We were a little surprised at the variations when it came to recharging:
Just be careful: Griffin didn't change the PowerBlock name when it updated the product to 2.1 amps for iPad. If it doesn't say "iPad", it's probably the older version that's currently selling on the cheap.
The poor performance of the MacBook Pro USB jack teaches us another thing: Direct juicing is better, if given a choice. Still, some USB wall plugs meant for iPhones and other mobile phones won't even support an iPad, so you may be stuck. If so, set aside several hours for a good power-up.
One last thing about iPad battery management: It's worth taking a peek at Apple's recommendations for optimal iPad battery lifespan. Of note, you ought to take your iPad through a full recharge cycle once a month. That means running it down completely, then charging it to 100 per cent. It's actually the first part that's hardest, as we found out while trying to test all of these charging techniques. [Giz iPad Coverage]