There is a lot more to this than meets the eye.
USB powering depends on the device being powered.
When telephones started charging on USB ports, they used no more than 1/2 amp which was about what the USB 1.0 spec provided.
Then along came tablets that now can draw a little over 2 amps which exceeds even what the USB 2.0 spec provides for power. So the tablet wall wort charges have some circuitry in them to figure out that a tablet is plugged in and it will supply the full current for "rapid" charging, but when you plug the tablet into a normal USB port on your computer, the tablet only trickle charges. Unfortunately the tablet makers don't have a standard for the full power sensing circuitry so a charger for one tablet may not work to charge another brand of tablet, even though the charger socket is a USB connector. You must use a proprietary cheater cable for the tablet of choice to get high powered charging, but at least they are inexpensive and you can get them at Amazon, CSquid cables.
It is not a trivial task to design an inexpensive 2 amp USB charger. I have designed one, but it is expensive to make in small quantities. I don't advertise them anymore, but you can see them here:
www.commitlift.com/usbchargers.html I made a couple of them for my son's RV-7 and can even supply them mounted in standard sized panel cutouts. The advantage of these is that they attach to your panel with two screws and use heavy duty USB port hardware, which is one reason they are expensive. The other reason is the circuit that converts 12 volts to 5 volts efficiently, is not a trivial and inexpensive device. If you are interested PM me.