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, video card, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor and speakers into a separate outlet, though.) We measured how each of our test systems used power during a roughly one-minute period, during which time we executed 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.

Complete results are available in our Core 2 Extreme QX6800 review here, but for this article, we're looking at 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.

You'll notice that we've not included the Athlon 64 FX-72 here. That's because our "simulated" FX-72 CPUs are underclocked versions of faster processors, and we've not been able to get Cool'n'Quiet power-saving tech to work when CPU multiplier control is in use. We have included our simulated Core 2 Duo E6600 and E6700, because SpeedStep works fine on the D975XBX2 motherboard alongside underclocking. The simulated processors' voltage may not be exactly the same as what you'd find on many retail E6600s and E6700s. However, voltage and power use can vary from one chip to the next, since Intel sets voltage individually on each chip at the factory.

The 1/microjoules value in our power efficiency per dollar graph is really 1/(watt-seconds/1000000), or 1,000,000 m^-2 kg^-1 s². That's a little obscure, but it quantifies power efficiency in a readable fashion based on the source data, which is in joules. We're looking at power efficiency per dollar.

Despite having nearly the highest render energy of the lot, the X2 3600+'s bargain basement price keeps it atop the power efficiency per dollar standings. The E4300 claims second place, offering higher power efficiency and a lower price than the 4400+. Our bronze medal winner is the E6400, whose power efficiency is quite substantially above the E6300's, despite the small pricing gap between the two chips.

That said, we couldn't get away without mentioning the Core 2 Quad Q6600, which tops the power efficiency scale despite its fairly reasonable price tag. If you do 3D rendering work for Al Gore, this is the chip to get. Still, it's worth pointing out that the Q6600 doesn't have the lowest idle power consumption (see our full results here), so the X2 3600+ may yet be the friendliest to your power bill if you don't run compute-intensive tasks very often.