I am a big fan of using a fixed core voltage combined with the C3/C6 sleep states. This is all you have to do to minimize power consumption and you also end up with great performance too without having to fool around with setting just the right offset voltage.
The screen shot is from a Core i5-3570K running somewhere around 4000 MHz to 4200 MHz. My individual cores are able to spend over 99% of the time in the C6 sleep state. In my original testing, putting the entire CPU package into the deeper sleep states was not resulting in any power savings at the wall so I turned those off. That might have been due to an early bios issue so I plan to retest that in the near future.
The voltage reported by CPU-Z, etc. when a CPU is in the C6 sleep state becomes irrelevant so there is no downside to using a fixed voltage. It makes it a lot easier to dial in a quick overclock and when idle, over 99% of the time this CPU is running at close to 0 volts because the cores are in C6.
Ivy Bridge works great and Intel has made some big improvements to how fast a CPU core can transition from C6 back to the full power C0 state. You are far better off setting your CPU up like this than trying to save power by disabling these sleep states and then forcing your CPU to use a low multiplier when idle. All that does is it uses more power and reduces off idle responsiveness.
Putting a CPU core to sleep is the best way to save power. When using the Intel GPU and overclocked to 4200 MHz, an entire computer can draw as little as 35 Watts from the wall at idle when set up correctly. Buy a Kill-a-Watt meter and use the latest version of RealTemp to dial in your sleep states. I will try to post some more results and power consumption numbers when I get the chance.
RealTemp T|I Edition
I recommend the Windows High Performance profile. When you have C6 enabled, you are better off letting the CPU manage itself. Ivy Bridge does a far better job than Windows when maximizing performance and minimizing power consumption. Cores that are spending most of their time asleep also run nice and cool at idle, even when using the crappy OEM heatsink.
http://img266.imageshack.us/img266/8822/c6sleep.pngEdited by unclewebb - 2/2/13 at 9:38am