We're pretty pleased with the nice, low power consumption numbers our new test rigs are capable of producing at idle. Not bad for quad memory channels, Sandy Bridge Extreme, and an 850W PSU, eh?
Although the entire system's power draw is part of our measurement, the display is not. The reason we're testing with the display off is that the new Radeons are capable of going into a special ultra-lower power mode, called ZeroCore power, when the display goes into standby. Most of the chip is turned off, and the GPU cooling fans spin down to a halt. That allows them to save about 12W of power draw on our test system, a feat the GTX 680 can't match. Still, the 680's power draw at idle is otherwise comparable to the 7970's, with only about a watt's worth of difference between them.
We're running Skyrim for this test, and here's where Kepler's power efficiency becomes readily apparent. When equipped with the Radeon HD 7970, our test rig requires over 40W more power under load than it does when a GeForce GTX 680 is installed. You can see why I've said this is the same class of GPU as the GeForce GTX 560 Ti, although its performance is a generation beyond that.
Since we tested power consumption in Skyrim, we can mash that data up with our performance results to create a rough picture of power efficiency. By this measure, the GTX 680 is far and away the most power-efficient performer we've tested.
Noise levels and GPU temperatures
Even though the Radeon HD 7970 can turn off its cooling fan when the display goes into power-save, it doesn't convey any measurable advantage here. The GTX 680 essentially adds nothing to our system's total noise levels, which consist almost entirely of noise from the (very quiet) CPU cooler.
Under load, the GTX 680's cooler performs admirably, maintaining the same GPU temperature as the 7970 while generating substantially less sound pressure. Of course, the GTX 680's cooler has quite a bit less power (and thus heat) to deal with, but Nvidia has a long tradition of acoustic excellence for its coolers, dating back to at least the GeForce 8800 GTX (though not, you know, NV30.)
We're not terribly pleased with the fan speed profile AMD has chosen for its stock 7970 cards, which seems to be rather noisy. However, we should note that we've seen much better cooling and acoustic performance out of XFX's Radeon HD 7970 Black Edition, a card with slightly higher clock speeds. It's a little pricey, but it's also clearly superior to the reference design.