The Wii power brick has an output of 12V with 3.7A. That makes a max of 44.4 watts.

The Wii U power brick has an output of 15V with 5A. That makes a max of 75 watts.

Many games "right now" on the Wii U have shown to use a max of about 32 watts, so just by watt usage, the Wii power brick would be enough, but that isn't taking into account so many other factors, one of which is the stability of the system using a different volt and amperage. Assuming that the Wii U is stable at 15V and 5A, would it even be possible to use the Wii power brick? Here are the conditions for any device with regards to using a different adapter...

1) Adapter must be within -/+5% of set voltage of the original

2) Adapter must have an amperage at minimum what the original offers

3) Connector tip of adapter must fit securely.

Assuming the connector was the same, you'd only need to check the voltage and amperage. Going from Wii U to Wii power brick voltage is a difference of -20%, far more than the -/+5% limit. Amperage is 1.3A lower than the original. Either case would demonstrate the Wii power brick could not be used on the Wii U.

tl;dr

The Wii power brick does not meet the needs of the Wii U.

http://lazerlight.x10.mx/ - Lazer Light Studios - Home of the MM2 PTC project