Why is my rMBP drawing twice as much power as my friend's when idle?

Mine is late 2012 and has 8GB RAM. His is late 2014 and has 16GB RAM. Both have an SSD (although of different size) and both are running Yosemite. We set the brightness to 6 pips each and turn off Bluetooth and the keyboard light.

Using Activity Monitor we both ensure that our %CPU is very low (<1%) with no visible apps running (apart from Finder and Activity Monitor). We then launch CoconutBattery and check out the Battery Usage therein.

After allowing a few minutes for the usage to settle down his MB idles at 3.8W. Mine idles at 7.6W, leading to his battery life being around twice mine.

What could be causing this disparity?