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.
Not much of a shock to anyone at this stage – the AMD hardware demands a lot more power at the socket – around 60 watts more under load than the overclocked GTX960. The TDP differences between AMD R9 380 and Nvidia's GTX960 already pointed to this outcome.