This is easy to believe, but incorrect. Rarely do HDDs consume more power than the processor. The disk industry has been remarkably effective over the years at keeping power consumption low.
A typical mid-range HDD today uses somewhat under 10 watts. Actual consumption depends on the type of drive, capacity and usage. Let's first look at how to measure consumption for this one component, and then see what the figures tell us.
Measuring HDD Power Usage
In my previous column, I discussed how to measure the power consumption of a system. In all cases, you want to measure consumption at the plug. That is how much power is being consumed in total. I pointed to one inexpensive means of doing this, which is to use the P3 Kill-A-Watt Electricity Usage Meter from P3 International.
The problem that immediately comes to mind is how to use this meter to test the power consumption of a single disk. No one is going to swap disks in and out of a desktop system to ascertain wattage. The solution is a product from the USB 2.0 Universal Drive Adapter from Newertech, which sells for $29.95 retail.
This kit consists of a small power supply to drive an HDD and a USB cable that can hook the HDD to a system. This cable supports ATA drives, including notebook and 3.5-inch drives, plus SATA drives. You plug the USB cable into your system and into the hard drive and attach the drive to its power supply. Suddenly the HDD is running. It appears to your system as a plain old USB drive. By plugging the adapter's power supply into your watt meter, you can see power consumption by this one drive. And by having the HDD hooked to your computer, you can make the drive active or let it spin undisturbed.
Apart from the power measurement considerations, this set-up is useful for peering at the contents of old HDDs without having to mount them into your system. This use is the principal use intended for this adapter.
I recently ran some parallel ATA (a.k.a IDE) drives through this set-up, and they registered around 5 watts when the disk was spinning but not doing I/O, and at around 8 watts during I/O. When compared with the 40 watts to 100 watts consumed by the processor, it's clear that despite being mechanical devices, HDD are remarkably power efficient, even in the bad old days before power consumption was on anyone's radar drive.
I pulled out an old HDD from 1995 and it consumed only 13 watts when just spinning. If you measure consumption in watts/terabyte rather than pure watts, then the progress vendors have made in the last ten years is off the charts, although this is mostly due to their abilities to increase capacity.
SATA drives run at roughly 1 watt more in quiet and I/O states, and SCSI drives run at typically 2 watts more than the ATA numbers.
To come up with at unified number for a given disk drive, the formula that is used is:
Typical power consumption = idle * .90 + write * .025 + read * .075
Since you probably won't be able to distinguish read from write in terms of power consumption (although these numbers are available from vendor specifications), you can use this formula as a basis:
Typical power consumption = idle * .90 + I/O * .10
While it's clear that hard drives should rarely be the principal focus for energy conservation on desktop systems, servers and storage boxes, their figures become more important.
Because of this, HDD vendors are stepping up their energy conservation efforts. Part of the motivation for this is the release of ENERGY STAR 4.0, which establishes low-power consumption thresholds and no vendor wants to be rejected because its components push a system over the tight baseline requirements.
In this regard, Western Digital -- one of the leading drive makers -- is moving to a new product line called GreenPower. The company claims that its 1TB SATA drive runs at 4 watts when idle and 7.5 watts during I/O (I am expecting a drive to look at and will report on this if there are variances). This would raise the bar for power efficiency on desktop drives. I expect SCSI drives will eventually be on that same power curve as well.
Save Power: Hibernate
All of this means that power consumption is rarely the correct primary factor for upgrading HDDs. Secondly, if you do upgrade your HDDs, you'll get a small lift in absolute power savings and an enormous lift in watts per GB. The best way to save power on your disks, especially on desktops, is to use the built-in power reduction technology in your operating system that puts HDDs into hibernate mode following a given portion of inactivity. Per vendor specs, this move will do the most to save energy by cutting it roughly in half for the time spent asleep.
For more information on power consumption by specific drives, visit digit-life.com.
Andrew Binstock's blog on software and technical matters can be found at http://binstock.blogspot.com.