Vampire power: unplug the cable box!

The other day I became curious about how much power was being used by various electrical appliances in my house.  In particular, I was curious about “standby power” or “vampire power”: the electrical power drawn by devices when they are idle or even completely turned off.  So I stole borrowed a digital multimeter from work, connected it to an extension cord (had to cut open the cord), and voila–my own power meter!  To get the power usage in Watts, I measure AC current (amps, rms) and multiply by 120 volts.

The results surprised me!

First, the good news: the little power supply that I use to recharge my cell phone is actually quite efficient.  With no phone plugged into it, it draws only 0.0009 amps, for a power consumption of about 0.1 Watt.  At 10 cents per kilowatt-hour, that works out to about 10 cents per year.  I still unplug it when not in use, just out of habit, but leaving it plugged in clearly wouldn’t ruin either me or our planet.

Next, the bad news.   Many other power bricks were very inefficient, drawing 4 to 6 Watts even when hooked up to no load!  Other appliances clearly have similarly inefficient power supplies inside them, like my Braun coffee maker, which draws 3.5 Watts even when it’s turned off–all to run a stupid little LCD clock which I’ve never even bothered to set to the correct time.   Other losers were a boom box (6.8 watts when off) and a pair of computer speakers (7.9 watts when off).

Why are some of these so bad?  The answer, unfortunately, seems to be just bad design.  Old style power supplies often use transformers to step down the voltage from the 120 volts in a standard outlet to the handful of volts needed for the appliance.  These are the large, heavy black bricks that often feel warm to the touch when they are plugged in.  By contrast, the small power brick for my cell phone feels very light; it uses silicon-based electronics instead of the heavy iron-core transformer, and is much more efficient.  When the phone isn’t plugged in, the power supply doesn’t feel warm at all.

But the single worst offender was my cable box, made by Motorola and supplied to me by Comcast.  When it’s on, it draws 35 Watts.  But when it’s off, it still draws 34.5 watts! That’s costing me an extra $30 per year, for absolutely no benefit to me.  That’s  unconscionable!

The solution is simple: I have now put several of the worst offending devices on power strips with off switches.  Now when they’re off, they’re really off.  🙂  With a small amount of effort on my part, I should easily be able to save about $70 per year, which of course also reduces my carbon footprint and is generally good for the planet.

Do you have a story or question about “vampire power” you’d like to share?  Leave a comment and let me know.

Published by


Matt Trawick holds a B.A. from Oberlin College and a Ph.D. from Ohio State University. He was a postdoctoral fellow and later a lecturer at Princeton University before coming to the University of Richmond as an assistant professor in 2004. His research interests include block copolymers, nanotechnology, and atomic force microscopy.

15 thoughts on “Vampire power: unplug the cable box!”

  1. I actually don’t know how widely read this blog is. It would be interesting to count page accesses sometime. Thanks for reading!

  2. Very interesting post! I have one question though: for your cable box, where does all the power go when it’s off? I mean the current is either heating up something or radiating EM waves, right? Also the ratio of 34.5/35 is intriguing: when it is on at 35W, does it suggest that most of the power is doing nothing useful?

    And, obviously, students do read the blogs time to time 🙂

  3. Interesting questions, Jeff!

    First off, it turns out that it’s not quite true, as I originally wrote, that the cable box is “absolutely no benefit” to me when it’s turned off. In fact, even when “off,” the cable box is downloading some digital information for its onscreen program guide–so that when I flip to channel 39, I see at the bottom of the screen that for the next 30 minutes I would be watching a rerun of “COPS” or whatever. That said, 34.5 Watts is an obscene amount of power for such a trivial task. By contrast, my cell phone on standby is also in constant digital contact with local towers, and draws perhaps tens of milliwatts.

    Many other devices, like power bricks, draw power even while performing no useful function whatsoever. While some of the power goes to electromagnetic radiation, most of it goes to resistive heating, making them warm to the touch. The situation is actually a little more complicated, however: there’s a distinction between “real power” and “apparent power” to a device, for highly capacitive or inductive loads. (See for a decent explanation of this.)

  4. Very clever, love that term – “Vampire Power.”

    This has bugged me for so long!

    I make a habit of unplugging any and all devices not in use that seem to be drawing power.

    What a waste of energy … I agree, most of this waste is due to bad design.

    Yes, we’re reading – thanks for the great post.


  5. Interesting post. Had no idea how much power a cable box could use – even when not in use! I’m the kind of person who just leaves everything plugged in, but now I’m rethinking that whole strategy.

  6. Interesting post. This likely explains why I have such a high electric bill even when I think I’m conserving power by shutting my equipment off.

    Also, if you’d like to track your “hits” on this site checkout You simply get a piece of code for this site, install the code as per the instructions, and you’ll see exactly how many gets (one piece of code for each page you want to track).

    Keep up the good work!

  7. Thanks for the info I had no idea that idle appliances could be so expensive to run.Must remember to unplug!!!

  8. I completely agree that having appliance that are not on but plugged in do use a lot of energy. I did the same test and unplugged my cable box, tv and dvd player everyday to see if it would make a difference. My bill was much lower the next month.

  9. Nice post. Just moved TV, speakers, box, DVD to a strip to turn it off. Thanks

  10. Interesting post. This could be the reason why my electric bill is so expensive even when I think I’m conserving power by shutting my equipment off.

    Webmaster of Dirt Bikes for Kids

Comments are closed.