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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.

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.

Wow, the Core i7's idle power consumption is very reasonable, especially considering it has a third DIMM in the system that the others don't.

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.

The Core i7's peak power use is definitely up from the quad-core Penryns, as one might expect from a larger chip with a design focused on keeping execution units more fully occupied. Peak power draw is only part of the story, though.

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.

The mix of reasonably low idle power draw and relatively short render times adds up to a moderate amount energy consumed by the Core i7 systems over the duration of our test period.

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.

Although the Core i7 systems tend to consume a little more power at peak than the quad-core Penryns, they also tend to finish rendering much sooner. Their more efficient execution means the Core i7 processors require less energy to complete the task of rendering our sample scene. These results also illustrate why Intel claims Hyper-Threading improves power efficiency—because it can.