Hi guys, question about electricity bill costs here.

Alienware just released a new SFF pc (Alpha R2) that has the GTX960 that is about RM400 more expensive (after conversion) than an equivalent SFF rig I plan to build with a Saphire R9 380 ITX compact card. The alienware is smaller and uses an external 130W PSU, whereas my build will be using a 620W PSU.

As I'm a student who will be paying my own electricity bills, how big of an impact will the Wattage difference impact the bills? On paper my SFF mini ITX build would be 5x more power hungry than the Alienware console, which sounds like the money I save on electricity might mean that the alienware is actually more worth it (everything else being equal) Is my reasoning sound though? I've no idea how to calculate impact of PSU's on bills