Power consumption and efficiency
We used a Yokogawa WT210 digital power meter to 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. (The monitor was plugged into a separate outlet.) 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.
We'll start with the show-your-work stuff, plots of the raw power consumption readings. We've broken things down by socket type in order to keep them manageable.
For space reasons, we've only included the most relevant portion of the CPUs we tested in the plot below. For the full slate of power-over-time plots, see our Sandy Bridge review. Also, you'll see that we have two sets of results for the Atom- and E-350-based systems. The primary set comes from a configuration with a discrete GTX 460 graphics card and our 610W standard PSU connected. The second set was captured with only integrated graphics in use and our laptop-style power brick supplying the juice.
This plot gives us our first indication of the E-350 APU's power draw and performance both. As you can see, the Atom and the E-350 setups use fewer watts under load than any of the conventional desktop CPU-based systems do at idle. Switching our PSUs and graphics solutions drops power consumption ever further, well below 30W for both the Atom and Zacate systems. The E-350 and the Atom D525 track very closely together in power draw overall.
As for performance, we've had to extend our time window for completion of the rendering task substantially in order to accommodate the low-power processors. Even the Core 2 Duo E6400 takes roughly half the time to finish the job. The E-350 finishes ahead of the Atom D525, but not by a lot.
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 each CPU has completed the render. 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, when the processors were rendering.
Here's a clearer view of idle and peak power consumption. Check out the E-350 system pulling only 14W at idle on the brick PSU. Whoa. The E-350 is a pretty close match to the Atom D525 under load, which is no bad thing. Amusingly, the Pentium EE 840 system draws nearly ten times as much power under load as the E-350 one does. Atom and Zacate are unquestionably in a different weight class than the desktop CPUs, a fact we'll want to remember so we can keep the forthcoming performance results in perspective.
We can pinpoint efficiency more effectively by considering the amount of energy used for the task alone. 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.
If you think of energy efficiency in terms of power draw at the wall socket alone, the E-350 and Atom are our obvious champions. However, once you consider more targeted measures like the energy required to complete a task, today's regular desktop processors can be surprisingly efficient. The long completion times of the Atom and Zacate systems put them at a disadvantage. Nevertheless, the E-350 on the brick PSU claws its way to the middle of the pack, several notches above the Atom D525 system.