Apparently from the shot behind the box, which says on a sticker "Ex-W" and under it "45". However, even if that has something to do with PSU capabilities, it's partly ripped sticker, and could just as well be 145 as 45, the sticker is ripped in a manner which would hide anything before the number 4.
http://www.joystiq.com/2011/06/14/plenty-of-close-up-wii-u-hardware-photos/
Thanks. Well, for reference the Wii power supply delivers 52W. And those kinds of power sockets are typically labelled with accepted voltage rather than power. I hereby express doubts.
Has anyone been able to determine that these boxes were actually functional? (Not that it matters a lot since the silicon of the real product isn't available yet.)