Power consumption and efficiency
Our Extech 380803 power meter has the ability to log data, so we can capture power use over a span of time. The meter reads power use at the wall socket, so it incorporates power use from the entire system—the CPU, motherboard, memory, graphics solution, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor into a separate outlet, though.) We measured how each of our test systems used power across a set time period, during which time we ran Cinebench's multithreaded rendering test.

All of the systems had their power management features (such as SpeedStep and Cool'n'Quiet) enabled during these tests via Windows Vista's "Balanced" power options profile.

I've whittled down these results to just the new processors being tested. You can see the results for the other processors in prior reviews.

Let's slice up the data in various ways in order to better understand them. We'll start with a look at idle power, taken from the trailing edge of our test period, after all CPUs have completed the render.

One questions pops out immediately as we look at these results: Why does the Core i7-975 Extreme draw so much more power at idle than the 965 Extreme? Chalk it up to our new motherboard. The Gigabyte EX58-UD5 simply draws more power than the Intel DX58SO that we used with the other Core i7 processors. Gigabyte has a purported solution for this problem in the form of its Dynamic Energy Saver Advanced utility, which is supposed to reduce power consumption. I spent some time trying various versions of this utility, including the latest from Gigabyte's website, on the EX58-UD5, and none of them worked—they were somehow incompatible with the BIOS revision I was using (the latest publicly available). Frustrating. I had expected Gigabyte to have this issue sorted by now.

Beyond that one issue, the rest of the CPUs tested look well within expectations. Notice that with its two disabled cores still present, the Phenom II X2 550 consumes as much power at idle as any AMD quad-core processor. Still, the Phenom II's idle power draw is respectably low.

Next, we can look at peak power draw by taking an average from the ten-second span from 15 to 25 seconds into our test period, during which the processors were rendering.

Intel has the edge here in both the value dual-cores and the mid-range quads. The 975 Extreme again draws more power than the 965, probably largely due to the motherboard.

Another way to gauge power efficiency is to look at total energy use over our time span. This method takes into account power use both during the render and during the idle time. We can express the result in terms of watt-seconds, also known as joules.

We can quantify efficiency even better by considering specifically the amount of energy used to render the scene. Since the different systems completed the render at different speeds, we've isolated the render period for each system. We've then computed the amount of energy used by each system to render the scene. This method should account for both power use and, to some degree, performance, because shorter render times may lead to less energy consumption.

These final results should be no surprise to anyone who has been paying attention. In multithreaded applications, multi-core processors are much more efficient. That's one reason server processors have been racing toward six and eight cores per socket. Even with its motherboard handicap, the Core i7-975 Extreme places among the top of the pack, because it spent so little time at peak utilization rendering the scene.

The rest of the results are close, yet Intel has a clear edge. The Q8400 proves more efficient than the Phenom II X4 940, and the Pentium E6300 requires less energy to render the scene than the two X2s.