There are no power outlets in any United aircraft in Economy. The relevant reference from the United web site is: http://www.united.com/page/article/0,6722,1074,00.html

The electrical outlets on the aircraft floor are for the cleaning crews. I do not believe you would obtain power from those outlets while the aircraft is in motion. Certainly, they are not advertised or promoted for that use.

It would not surprise me at all if someone were to attempt to use those outlets and a Flight Attendant observed that, they might presume you were tampering with the aircraft. Don't go there.

The First/Business outlets are the "Empower" type. Many adpaters are available that support the Empower standard. United's website reference, above, features the JUICE product from iGo. Others are also available.