How green is your PC?

How much does it cost to run a PC or a Windows Home Server 24/7? I've just completed a abttery of power management tests in my office, and the numbers surprised me. In my neighborhood, running a home server costs about $5 a month in electricity, but I can cut that bill by two-thirds just by using the default power management settings in Windows Vista.

In my review of HP's MediaSmart Server a few weeks back, some commenters asked how much power this always-on server uses. I promised to do some testing, which is now complete. In the process, I tested a desktop PC and learned a few things about Windows power management that I didn't know before.

To measure power consumption, I used a device called a Kill A Watt Electricity Usage Monitor, which is about the size of a standard plug-in surge protector. (I picked up a pair for under $25 each from a mail-order supply house. Amazon has them for $21 each currently.) You plug the gadget to be tested into the Kill A Watt socket and then plug the box into the wall socket. The front panel shows voltage, line frequency, power factor, and cumulative kilowatt-hour usage for that device.

The MediaSmart Server I tested contained a full complement of four internal SATA hard drives. Over an 18-day period, the server used a total of 31.3 kWh, for an average power consumption of 72.5 watts.

My home-built Windows Home Server box, a Dell E521 with two internal SATA drives and one external drive connected via eSATA cable, used 39.7 kWh over the same period, which works out to 91.9 watts on average. or about 26.7% more than the HP server. (This calculation includes the power consumed by the external drive's power supply but not by the monitor attached to the server, which is off except for rare maintenance operations.) If I added a second external drive to match the configuration of the HP server, this value would probably go up slightly.

My electric company charges $0.0788 per kWh. That means the MediaSmart server costs me $4.17 per month to run. Over the course of a year, that means I save about $13.39 by using the smaller, more energy-efficient device rather than building my own. And even though this is an always-on device, I think that Microsoft could probably do more to conserve power in Windows Home Server, and I hope they'll do exactly that in the next upgrade.

I also attached a Kill A Watt meter to the Dell C521 PC that I've been using for my ongoing Media Center experiments. At rest, it uses about 64 watts, and its power consumption is roughly equivalent to the HP server over time. However, it's dramatically more power-efficient, thanks to Windows Vista's sleep mode. In the past 24 hours, it has used less than 0.5 kWh. Over the course of a month, that's about $1.20 in electricity. The secret of its power-saving success is S3 sleep mode. When this system kicks into S3 mode, it uses a mere 3 watts, according to the Kill A Watt device. That 0.5 kWh equals 8 hours a day of full-power usage, coupled with 16 hours in sleep mode. If I were to leave it on with sleep disabled, energy usage would triple. Using the default Balanced power settings for the three PCs in this house will save more than 1000 kWh over the course of a year, or $82.

After taking these measurements, I'm sold on the benefits of using sleep mode in Windows Vista. I'll explain how to fine-tune power settings (and how to avoid one big gotcha) in my next post.