At home I am powering the device from the USB port on the TV. It powers it just fine.
I have a wifi dongle plugged into one port and a USB Gamepad on another.
I brought it into work to show it off and used a 2a wall wart and otherwise the same setup.
I don't need the wifi adapter when only using Retropie but I pretty much leave it in all the time.
Edimax EW-7811Un WiFi dongle.
DoTop Generic Super Nintendo Controller
22inch Raspberry Pi power cable with switch.
Generic 2a 5v wall wart.