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' "Balanced" power options profile.

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.

This is where Lynnfield's integration pays off. The Core i5-750 and i7-870 systems idle at roughly 20W lower than the Core 2 Quad or Phenom II X4 systems. The reduction from the Bloomfield system is even more dramatic.

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 Lynnfield chips' power consumption under load isn't quite so tame—there's quite a bit of dynamic range in these designs in terms of power draw. Still, you can probably already surmise that no competing processor offers as much performance for the power consumed.

We can quantify 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.

We can quantify efficiency in an even more focused manner 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.

The Core i5-750 and Core i7-870 trade places at the top of the leaderboard in these last couple of tests. However you choose to quantify it, though, these Lynnfield processors are the most power-efficient desktop CPUs around (outside of low-power specials, of course), and overall, it's not even close.