Coding Horror

programming and human factors

The Cost of Leaving Your PC On

Between my server and my Windows Media Center home theater PC, I have at least two PCs on all the time at home. Have you ever wondered how much it's costing you to leave a computer on 24 hours a day, 7 days a week?

The first thing you need to know is how much power your computer draws. The best way is to measure the actual power consumption. You'll need a $30 device like the Kill-a-Watt to do this accurately. Once you get one, you'll inevitably go through a phase where you run around your home, measuring the power draw of everything you can plug into a wall socket. For example, I learned this weekend that our 42" plasma television draws between 90 watts (totally black screen) and 270 watts (totally white screen). Based on a little ad-hoc channel surfing with an eye on the Kill-a-Watt's LCD display, the average appears to be around 150 watts for a typical television show or movie.

But I digress. Once you've measured the power draw in watts (or guesstimated the power draw), you'll need to convert that to kilowatt-hours. Here's the kilowatt-hour calculation for my server, which draws ~160 watts:

160 watts * (8,760 hours per year) / 1000 = 1401.6 kilowatt-hours

The other thing you'll need to know is how much you're paying for power in your area. Power here in California is rather expensive and calculated using a byzantine rate structure. According to this recent Mercury News article, the household average for our area is 14.28 cents per kilowatt-hour.

1401.6 kilowatt-hours * 14.28 cents / 100 = $200.15

So leaving my server on is costing me $200 / year, or $16.68 per month. My home theater PC is a bit more frugal at 65 watts. Using the same formulas, that costs me $81 / year or $6.75 per month.

So, how can you reduce the power draw of the PCs you leave on 24/7?

  • Configure the hard drives to sleep on inactivity. You can do this via Control Panel, Power, and it's particularly helpful if you have multiple drives in a machine. My server has four hard drives, and they're typically asleep at any given time. That saves a solid 4-5 watts per drive.
  • Upgrade to a more efficient power supply. A certain percentage of the input power to your PC is lost as waste during the conversion from wall power to something the PC can use. At typical power loads (~90w), the average power supply efficiency is a disappointing 65%. But the good news is that there's been a lot of recent vendor activity around more efficient power supplies. The Fortron Zen fanless power supply, for example, offers an astonishing 83% efficiency at 90w load! If you upgraded your power supply, you could theoretically drop from 122w @ 65% efficiency to 105w @ 83% efficiency. That's only a savings of $20 per year in this 90w case, but the larger the power usage, the bigger the percentage savings.
  • Don't use a high-end video card. I'm not sure this is widely understood now, but after the CPU, the video card is by far the biggest power consumer in a typical PC. It's not uncommon for the typical "mid-range" video card to suck down 20+ watts at idle -- and far more under actual use or gameplay! The worrying number, though, is the idle one. Pay close attention to the video card you use in an "always-on" machine.
  • Configure the monitor to sleep on inactivity. This one's kind of a no-brainer, but worth mentioning. A CRT eats about 80 watts, and a LCD of equivalent size less than half that.
  • Disconnect peripherals you don't use. Have a server with a CD-ROM you rarely use? Disconnect the power to it. A sound card you don't use? Pull it out. Redundant fans? Disconnect them. That's only a savings of a few watts, but it all adds up.

If you're building a new PC, it's also smart to avoid Intel's Pentium 4 series, as they use substantially more power than their AMD equivalents. Intel's Pentium-M, on the other hand, delivers the best bang for the watt on the market. Although it was originally designed for laptops, it can be retrofitted into desktops.

Written by Jeff Atwood

Indoor enthusiast. Co-founder of Stack Exchange and Discourse. Disclaimer: I have no idea what I'm talking about. Find me here: http://twitter.com/codinghorror