I wrote about my testing of notebook disk drive power usage on ZDnet yesterday (see How much does a flash disk increase battery life?). I pulled the 160 GB WD Scorpio out of my MacBook and ran it on wall power through a Kill-a-watt meter to better understand power usage. I learned – or relearned – a few things.

What surprised me most was the fact that as I measured power usage I saw that I/O, CPU and network usage were all intertwined. I’m surprised I was surprised since they are systems and the pieces work together.

I/O and CPU
I ran a defrag program that exercised the disk while driving CPU usage on a Core Duo to 90%. Since the CPU uses almost 3x the power of the disk it is the CPU and not the disk that is the power hog.

In fact, the biggest power hog is the base system: 13 watts sitting there doing nothing, LCD, Wi-Fi and Bluetooth all turned off, with no CPU load.

Hey, Taiwan, want to build long-battery life notebooks: figure out how to turn more pieces of the system off when not in use.

28 watts max
With the LCD turned on full, Wi-Fi and Bluetooth on, and the CPU at 90% the diskless notebook pulled 28 watts.

Turning on Wi-Fi was good for 2 watts, almost as much as a busy disk, partly due to the CPU load of running the TCP/IP stack.

Optical drives are power hogs too. Maybe not the drive itself, but the associated graphics or CPU processing.

Here are the numbers

The StorageMojo take
The significance of the intertwined nature of I/O, CPU and network usage is this. Flash drives sip power, but in a busy system it is all the other subsystems that chew up the battery.

The power saving advantages of a flash drive are best in a lightly loaded system with a long battery life, i.e. your cell phone, PDA or ultra-light notebook. In a 2-3 hour battery life 15-17 inch notebook the 2-3 watts a disk uses is almost noise level.

I see some evidence that the flash drive makers are adjusting their marketing to these facts. That is all to the good. Given the price/capacity differential you want flash disk customers buying for the right reasons.

Comments welcome, of course.