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.
Interestingly, this Gigabyte 1660 Ti draws 10W less than the higher-clocked MSI Gaming X model, meaning it is actually less power hungry than GTX 1060 – despite the massive performance increase. I said as much in my review of the Gaming X, but Nvidia is clearly streets ahead of AMD in the power efficiency race.