Both the 777-200 and 777-300 have universal power outlets in all classes (will fit both a US and UK plug). They all operate at 110V AC. I always travel (and recommend traveling) with a small 3-prong power strip, as I've experienced that 2-prong devices can easily fall out of the plugs, especially when it's turbulent - this is particularly relevant for devices that don't have a battery attached (such as the DVD player you are planning to bring), so your show isn't interrupted if the plug falls out (plus you can then plug in more than one device or share with a neighbor, as if my memory serves me correct, there are only outlets between the seats (so 2 outlets for 3 people, 3 outlets for the middle 4 people).