Nintendo specifically said it was not
portable, as much as the controller seems to be.
The controller gets all of its information through Near Field Communication
streamed from the console. All of the processing happens in the Wii U console, would be sitting somewhere near your TV, connected via HDMI, and plugged into an AC outlet.
The controller acts as a secondary display and input device. Like any other wireless controller, if you go out of range, it won't work.