The GTX 660 consumed 6% less power than the GTX 660 Ti under load with a total system consumption of 292 watts. Nvidia's new card consumed 6% more power than the HD 7850 consumed under the same conditions, but it also delivered 10% more performance on average (13% faster in Crysis 2, the game we used for our stress test).
However, compared to the GTX 660 Ti, the GTX 660 doesn't fare quite as well -- at least not at first glance. The latter consumed 6% less power while being 14% slower on average. That said, in the game tested (again, Crysis 2), the GTX 660 was just 5% slower than the Ti version, so both cards seem similarly efficient.
The Gigabyte WindForce 2X cooler worked its magic, keeping the GTX 660 at 61 degrees Celsius under full load using FurMark. This is a few degrees cooler than the HIS Radeon HD 7850 IceQ and roughly the same temperature as the Gigabyte Radeon HD 7950 WindForce 3X.