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 our LAME MT multithreaded MP3 encoding test (using the executable from the Microsoft compiler).

Interesting! Let's slice up these 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 encoding task.

The overall platform power use of the Atom 230 and Nano L2100 are very similar to one another, with less than a watt separating them.

Next, we can look at peak power draw by taking an average from the 15-second span from 15 to 30 seconds into our test period, during which the processors were all busy.

The Atom and Nano are both true to their TDP ratings. The Nano system's power use shoots up by over 20W when the CPU is busy, while the Atom system's power draw increases by less than two watts. Intriguingly, the Pentium M 760's peak power consumption turns out to be lower than the Nano L2100's. Of course, the L2100 is aimed explicitly at desktops, but I still hadn't entirely expected this outcome.

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 encoding work and during the idle time. We can express the result in terms of watt-seconds, also known as joules.

We can quantify efficiency even better by considering the amount of energy used to complete this task. Since the different systems completed the encode at different speeds, we've isolated the work period for each system. We've then computed the amount of energy used by each system to encode the file. This method should account for both power use and, to some degree, performance, because shorter encode times may lead to less energy consumption.

The Nano L2100 and Atom 230 take very different paths to completing the task with almost the same amount of energy consumed. The Atom takes quite a bit longer finishing, but keeps its power draw vastly lower as it works. The Nano consumes more power, yet finishes the work over a shorter period of time.

Looking at these results, one can't help but think that the Atom could be an astoundingly power-efficient processor when coupled with a chipset and platform with a lower power use floor. Intel, of course, has such things in the works for other markets. In the same vein, we're definitely at the ugly end of the clock frequency/voltage curve with the Nano L2100. The Nano U2400, which runs at 1.3GHz and has an 8W TDP, ought to offer much better performance per joule.

As it stands, though, the Pentium M 760, an older chip manufactured on a 90nm fab process, used markedly less energy to encode an MP3 than either of the low-cost platforms we're testing today—a testament to the remarkable energy-efficient performance of the Dothan Pentium M design.