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.
The really interesting thing is that power draw hasn't changed compared to the reference card. Both average 222W over three games, despite the Nitro+ running at a higher clock speed. The Silent BIOS is slightly more frugal, but only draws 10W less.