What Enabling C6/C7 Low-Power States Do on the Core i7-4770K Haswell CPUFri, Jun 14, 2013 - 12:00 AM
C6 and C7 Power States
On our Windows 8 test platform we were able to quickly see the benefits of running with the C6 and C7 power states enabled. We found that with C6/C7 disabled that the system would idle between 23.7 and 24.1 Watts, but with them enabled we were seeing between 20.5 to 21.2 Watts. Both numbers are impressive as you have to keep in mind that the whole system is being measures here with a Kill-A-Watt power meter at the wall outlet. The Intel Core i7-4770K 3.5GHz quad-core processor might be the fastest LGA1150 processor that money can buy, but it also happens to be really efficient and by enabled C6/C7 low power states we were able to get ~3 Watts or nearly 13% power savings at idle!
With the Intel Desktop Board DZ87KL-T75K running optimal settings, meaning C6/C7 power states are disabled, the Intel Core i7-4770K was bouncing around 36-38C with the average being 37C. The Intel Extreme Tuning Utility shows that the CPU Total TDP is 5 Watts. Many people have complained about how hot Intel Haswell processors run, so it will be interesting to see if enabling C6/C7 will lower processor temperatures at idle.
Sure enough! By enabling ‘Lowest CPU Idle Power Setting’ in the BIOS the C6/C7 power states helped lower the CPU Core Temperature down to 34-36C with an average of 35C. This is a solid two degree Celsius temperature drop, which is rather impressive. Notice that the CPU Total TDP is now just 2W instead of 5W. This also coincides with the ~3 Watt performance difference that we noted at the wall.
Final Thoughts and Conclusions:
4th Gen Intel Core Processors C6/C7 low-power states are a feature that many people are going to miss out on since the vast majority of motherboard manufacturers have disabled it by default and that is a shame. Our testing showed a 13% reduction in power consumption and a measurable drop in the CPU Core temperature. If you have a high-end modern power supply that supports C6/C7 then you should most certainly turn it out and see what it does for your “Haswell” based system. The motherboard makers have it disabled as they don’t want to deal with the tech support calls that are sure to happen when someone tries to plug in a $15 generic power supply they bought off ebay six years ago and their computer crashes at idle.
Intel has released a PSU Selector to help people figure out if their power supply supports C6/C7 power states. The list is far from complete, but does show that some models from Corsair, InWin, Seasonic and FSP are working properly with C6/C7 power states. When you go look at Intel’s PSU Selector you are looking for support in the “12V2 Min-load 0A” column. Why must Intel be so confusing? They call them ‘C6/C7 low-power states’ on the marketing slides, in the BIOS it is called ‘Lowest CPU Idle Power Setting’ and then on the PSU support list they are called “12V2 Min-load 0A”. WTH? If it confuses us, then it is going to really confuse the layman that is most certainly not a PC enthusiast!
Legit Bottom Line: If you have a modern high-quality power supply that supports C6/C7 power states then you should try enabling it as it lowers your idle power consumption and even your CPU temperature!