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Power consumption and efficiency
We used a Yokogawa WT210 digital power meter to 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. (The monitor was plugged into a separate outlet.) 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.

We'll start with the show-your-work stuff, plots of the raw power consumption readings. We've broken things down by socket type in order to keep them manageable.

We can slice up these raw 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. 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, when the processors were rendering.

You already know that the Core i7-2600K is often substantially faster than the i7-875K across many of the applications in our test suite. Now consider that the 2600K-based system has lower idle power draw and requires 26W less under load than the 875K. The gains in power efficiency seem mind boggling—so let's quantify them.

We can highlight power efficiency by looking 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. (In this case, to keep things manageable, we're using kilojoules.) Note that since we had to expand the duration of the test periods for the Pentium EE 840 and Core 2 Duo E6400, we're including data from a longer period of time for those two.

We can pinpoint efficiency more effectively by considering the amount of energy used for the task alone. 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.

The Core i5-2500K system requires roughly 25% less energy to render our test scene than the Core i7-875K system does. That is a staggering improvement, even if our bar chart is a bit thrown off by the presence of the Pentium EE 840. To put a really fine point on the comparison between Intel's quad cores and AMD's, consider that the Core i7-2600K config needs under half the energy that the Phenom II X4 975 system does to accomplish the same work.