We measure the amount of power drawn from the wall by the entire test system. Our Core i7-5960X CPU is heavily overclocked and has a large bearing on the power draw readings when it is heavily loaded in GTA V (our chosen game for system readings). The important information is the difference in power draw levels between each card.
Power usage is still high for the R9 390X GPU, and Asus' STRIX card is no different even with its enhanced power delivery components and cooling solution taken into account.
The loaded system demanded almost 500W of power while playing GTA V. By comparison, the similarly performing GTX 980 STRIX-based system used over 100W less power.