We measure system-wide power draw from the wall while the card is sat idling at the Windows 10 desktop for 30 minutes. A reading under load comes from running the 3DMark Fire Strike Ultra stress test 20 times.
Nitro+ draws considerably more power than Red Devil, which is surely a consequence of the VRM design. PowerColor uses more phases and the benefit seems clear, as even the Nitro+ with its Silent BIOS pulls around 25W more than the Red Devil.