More details on the Wii U GamePad
The console streams video to the screen controller. Multiplayer will regularly involve one GamePad and several Wii Remotes (or other controllers).
However, according to Nintendo president Satoru Iwata, "When you use two, the frame rate halves to 30 frames per second." Presumably the company will fix this problem in time, as Iwata added, "At launch, we'll start by offering games that use only one [Wii U GamePad] and then expand from there."
Additionally, Nintendo officially confirmed that the GamePad's battery will only last 3-5 hours. Screen brightness and usage may decrease this estimate.
It will take 2.5 hours to charge a depleted battery.
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