wattage supply question for prebuilt computer vs homebuilt

Chicagonative230

Honorable
Oct 29, 2013
1
0
10,510
I'm looking at building a new gaming rig, but came across this ASUS computer (ASUS G10AC-US005S) and saw that it has everything I want included in it. My question: I've been reading up on building and everything I've read says you need at least a 650 watt power supply when you have a GTX 780 graphics card. How is this rig getting away with only a 500 watt supply? I know the new haswell processors are better on power use, but do they really make that big of a difference?