From FlyerGuide Wiki
EmPower is a 15 volt DC power system installed in all of United's international fleet and some of United's domestic fleet. The system provides DC power to passengers, allowing them to use laptop computers, DVD players, and other electronic devices with DC power inputs. Use of the EmPower system requires an adapter cord.
EmPower is available only in United First domestically, and not on all equipment. Domestic 747, 767, and some 757 and A320 aircraft are equipped with EmPower ports in First class. There is no difference in the equipment designation for EmPower equipped 757/320 and non-EmPower equipped 757/320 aircraft. There is no way to determine if a particular flight will be operated with EmPower equipped aircraft or not. Posters on FlyerTalk have tracked the tail numbers of equipped and non-equipped aircraft, however since there is no way to determine which specific aircraft will be used on a particular segment, this information is only useful when you are at the gate and can see the plane. On equipped aircraft, the empower socket will be located in the armrest.
757 p.s. aircraft are equipped with a power system which provides standard 120-volt AC outlets throughout the aircraft. United First Suites and United Business seats are provided one outlet per suite/seat. In the economy cabin, each set of three seats shares two outlets.
United First Suites and United Business seats on international 747, 777 and 767 aircraft are all equipped with one EmPower port per suite/seat. There is no EmPower provided in the economy cabin.
Aircraft which have undergone reconfiguration to the new premium international cabins provide power in the form of standard 120-volt AC sockets; no adapter is required. These sockets are available only in United First and United Business, there is still no power available to United Economy passengers.
In all cases where inflight power is available, each power outlet is limited to 75 watts. Some new laptop power supplies consume more than 75 watts; their use with the EmPower system may cause the circuit breaker to trip.
On p.s. and new international equipment no special equipment is required. Simply connect the standard AC charger to the 120-volt power outlet located in the suite or seat.
On all other aircraft, EmPower sockets are a special four-pin socket. An adapter cord is required to use the system. Most computer manufacturers sell power cords with an EmPower end. Third party adapters are also available, and may offer more flexability. Third-party cords typically allow connection to 120 Volt AC, 12 Volt DC from automobiles, as well as 15 Volt DC from EmPower, and feature a large number of connector tips, so that the same cable can be used with multiple electronic devices. One such product is the iGo Everywhere, which has received positive reviews on FlyerTalk.