The main reason for is because GPU’s usually spin up to full speed when the editor is open. You could buy a GTX 980 for it’s insanely low power-consumption 
My 980 sits at 48% usage with a scene open in UE4, but it does actually drop to 0% when UE4 is minimized. Previously, my 560 Ti would sit at 99% usage continuously, and would be practically on fire and draw considerably more current through the mains.