The DS, 3DS, Wii and Wii U all offered backward compatibility through hardware. I see no reason why they wouldn't do this on the NX. They will most likely make consumers by a Wii U Gamepad separately, just to keep costs low (for those users who don't have one).
So the answer is 75 Watts, the power draw of the Wii U.
Wii U is a 45nm chip..
If Nintendo went with a 16nm FinFET single chip (apu), they could probably cut that 75watts down by ALOT (like down to 20watts or so).
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071