This first measurement comes from when the display has been placed in power-save mode while the system is idle. When that happens, the newer Radeons invoke AMD's ZeroCore power feature and drop into a very low-power state, spinning down their fans in the process. That's why the Radeons draw so very little power here. Somewhat remarkably, the systems equipped with the reference and Zotac GTX 660 cards only draw about 4W more than the Radeon-equipped configs.
At idle, the GK106 draws very little power, although the Asus GTX 660 TOP pulls more juice than the other two cards. I guess weasel hair and mothballs can be pretty effective, huh?
When running Skyrim, the GTX 660 looks to be very efficient. The same system with a Radeon HD 7870 card installed draws 40W more than the Asus GTX 660 TOP.
Noise levels and GPU temperatures
Even though the Radeon HD 7000-series cards disable their fans entirely when the display is off, our measurements for them aren't much quieter than the Asus GTX 660 TOP card, probably because we're getting close to the ambient noise floor for our test room, which is nothing special in terms of sound isolation (unless you consider a basement special).
The reality is that many of these cards are quiet enough at idle that you'd barely notice them in the average room. We've seen real progress on this front in recent years, as the higher readings for the GeForce 200-series cards attest.
I knew the Asus GTX 660 TOP was quiet from using it and from the readings at idle, but its noise levels under load are something else entirely. Somehow, the card bends time and space, shaving 0.8 decibels off of its noise level at idle—and lowering the noise floor of the entire test system and environment at the same time. The really freaky thing is that this strange result pretty much matches my subjective impressions. I need to check the label on that new allergy medicine.
Oh, and the Zotac cooler is pretty good for its size. MSI's 7870 Hawk registers just a little higher on the decibel meter, which is a surprise, since it seems blessedly quiet subjectively.
Here's what an extra dose or two of fiddy cents buys. Asus' five-pipe cooler is magically quiet under load, yet it maintains some of the lowest GPU temperatures of the bunch. MSI's R7870 Hawk isn't far off on either front, but the Asus card has less heat to dissipate due to its more power-efficient GPU.