14.8v at 6400mah+ should get you acceptable battery life.
The issue with the Wii is it needs 13-14.8v upon start up then it can be run with less... I think it can go down to 10v after start up.(if not 10v, 11v is what it is.)
Edit: Whoops thought this was for the Wii not Wii U.
Wii U would need the same voltage(?) as the Wii.
Need a larger capacity battery. 12,000+ mah to get anything gaming done.
More likely you will need 20,000mah+ to get Okay battery life.
I will look at my Wii U power brick when I get a chance and give you a better estimation.