We measure system-wide power draw from the wall while running the 3DMark Time Spy stress test for 30 minutes.
We also use Nvidia PCAT to measure power draw of the graphics card only, with readings from both the PCIe slot and the PCIe power cables combined into a single figure. This provides us with significantly more accurate data to work with as it is measuring only the GPU power, and not total system power which is a fundamentally imprecise measurement.
Interestingly enough, graphics card-only power draw is 5W less than the Nitro+ and AMD's reference card. It's not clear why this is the case, but considering gaming performance is essentially identical between all three cards, it's certainly not cause for concern.