Power consumption and efficiency
Now that we've had a look at performance in various applications, let's bring power efficiency into the picture. 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.

Anyhow, here are the results:

Looking at the graph of the raw data, you can see that the QX6850 doesn't consume any more power at peak than the QX6800 did.

We can slice up the data in various ways in order to better understand them, though. 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.

With two chips and lots of transistors onboard, the QX6850's idle power wasn't going to be the lowest of the lot, but it isn't bad. Idle power is higher than the QX6800, perhaps due to the 1333MHz front-side bus.

Next, we can look at peak power draw by taking an average from the ten-second span from 30 to 40 seconds into our test period, during which the processors were rendering.

The QX6850's extra little bit of performance on the QX6800 requires no additional power at peak, and the QX6850 draws less power than a couple of the Athlon 64 X2s, as well. That's gonna leave a mark in Austin.

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.

Because the QX6850 finishes rendering quickly and drops back to idle, it doesnt' consume much power during our test time span. Those uppity Xeons finally get their comeuppance, too.

We can quantify efficiency even better by considering 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.

By delivering more performance with no additional power consumption, the QX6850 brings new highs in efficiency in this measurement, which may be our best means of quantifying the vaunted "performance per watt" of a processor. Betcha didn't expect to see a high-end part taking the lead in this regard. This is more evidence that Intel hasn't pushed the clock speed envelope terribly hard with its 65nm chips; even the high-end models are in a nice place on the power/speed/voltage curves.