Should I Turn Off My Computer At Night To Save Energy?

April 18, 2010

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Sure, go ahead.  You will definitely save a little energy.  The key word is that sentence is “little”.

First of all, computers aren’t the energy hogs that many people think they are.  The laptop I use at work has a 90 watt AC adapter, and the Mac Mini I have at home has a 100 watt AC adapter.  It’s important to note that these are maximum wattages.  Most computer usage doesn’t come anywhere close to the max, and when it does, it’s for short bursts.

Electricity usage is measured as a kilowatt hour (kWh).  The means that a device that has constant usage of 1000 watts would use 1 kWh every 60 minutes.  The cost per kWh varies, but is generally in the ballpark of 10 cents.  This means my Mac Mini costs about 1 cent per hour to operate when it is at its maximum consumption rate (100 watts = 1/10 kW).

But that’s not even what we’re talking about. I wouldn’t save the entire 0.1 kWh by powering off the computer.  Like most users, I’m going to put my computer into sleep mode at night.  Sleep mode typically uses between 1 and 5 watts.  In the case of my Mac Mini, 1.39 watts.  I wasn’t able to find this information on my exact model of laptop, but I suspect that its usage is similarly low, especially in “hibernate” mode.

Is it worth the effort to shut down your computer to save a few watts per hour?  Let’s assume that your computer uses a relatively high 5 watts when in sleep mode.  Let’s also assume that it is idle 75% of the time.  That’s 126 idle hours per week, or 6552 hours per year.  Multiply this by a usage of 5 watt per hour, and you get a savings of 32,670 watt hours, or 33 kilowatt hours.  Congratulations – you just saved $3.30.  Have a Mac Mini like mine, and the savings is just $1.25 per year.

This makes even less financial sense for businesses, which are paying their employees to shut down the computer at night and restart them in the morning.  If we assume 1 minute of lost productivity per day for 250 work days, this is more than 4 hours of lost productivity!  Unless you’re paying your employees less than 90 cents per hour, you lost money on this exchange (assuming the $3.30 in energy savings).

Of course, you could argue that every little bit of saved energy adds up, in terms of environmental impact.  This is definitely true – and if you want to conserve every little bit of energy, then go ahead and shut down the computer.  However, there are a lot of other changes that give you a lot more bang for the buck.  If you have a 60 watt incandescent bulb in a lamp you use an average of two hours per day and switch it out with an equivalent compact fluorescent bulb (using 13 watts), you save 47 watts per hour of use – or 34 kWh during the course of a year ((34 X 2 X 365)/1000).  Yep, taking a minute to switch out a bulb saves as much energy as shutting down and restarting your computer 365 times!

I’m not suggesting that we waste energy.  I’m not even suggesting that we shouldn’t power off computers and other unused electronics.  I’m simply suggesting that if you’re willing to put the effort into this very small energy savings, you might also focus some energy on things that save even more energy.

9 Comments (+add yours?)

  1. Evan @ 40Tech
    Apr 18, 2010 @ 15:44:16

    Great timing – my wife has been getting on me because I leave one of my machines running almost all of the time (but that’s because it has been acting strange lately, and sometimes loses its internet connection on reboot until I unplug and replug the Cat5e cable from the back of it). It has a 750W power supply, but is probably only using a fraction of that overnight.
    .-= Evan @ 40Tech´s last blog ..How to Use Your Scanner as a Fax Machine =-.


  2. kosmo
    Apr 18, 2010 @ 22:10:03

    @ Evan – I’m not sure if you mean that you have it on but asleep, or just on all the time. Even if the machine is “awake” but idle, it’s not going to use much power. Maybe a couple dozen watts at most.

    It’s not another bad ethernet cable, is it? 🙂


  3. poch
    Apr 19, 2010 @ 07:01:29

    I only shut off my laptop when the adopters are heating.
    Otherwise I put it on hybernate.


  4. Squeaky
    Apr 20, 2010 @ 07:37:27

    Hey Kosmo, great article! Will you try and come up with an analysis for monitors as well? My wife turns off the monitor but leaves her desktop running. I’m wondering if that is a money saver or not.


  5. kosmo
    Apr 20, 2010 @ 07:54:34

    @ Squeaky – Let me point you to Michael Bluejay’s site. He has done a nice analysis of computer-related electrical use, and there’s no point re-inventing the wheel.

    The short answer is that this WILL save energy.


  6. Evan @ 40Tech
    Apr 21, 2010 @ 08:54:41

    I’m not sure if it is cable related or not. There was one that didn’t work at all, but the few other cables I’ve tried to lose the connection sometimes when the PC is powered down. My next step will be to update the motherboard bios, or even install a PCI NIC.

    It doesn’t lose its connection when sleeping (well, if I recall), but I have this weird issue were one of the fans (heatsink fan, I think) blows at full blast and doesn’t stop when coming out of sleep. On my long term to-do list is to figure out what the heck is causing that, and/or get a quieter fan (it isn’t too quiet even when not blowing at full tilt).
    .-= Evan @ 40Tech´s last blog ..Springpad vs Evernote — Why It Might Be Better… =-.


  7. Evan @ 40Tech
    Apr 21, 2010 @ 08:57:26

    Oops, “to lose the connection” should be “do lose the connection”
    .-= Evan @ 40Tech´s last blog ..Springpad vs Evernote — Why It Might Be Better… =-.


    Aug 30, 2010 @ 22:05:39

    I used to leave my computer on for 24 hours everyday but when I started turning it off at night I was able to cut my electric bill in half. I don’t think that’s a little savings. 😀


  9. kosmo
    Aug 31, 2010 @ 06:55:46

    I’m glad you were able to slice your electric bill in half, but let’s take a closer look.

    1) Even if your computer was the sole electrical device in your house, and even it it consumed as much energy at night as it does during the day, the most saving you could expect to realize would be 33% (assuming it is turned off for 8 hours and on for 16).

    2) My own computer cannot consume more than 67.2 kwh per month, even if it were rendering video files and burning DVDs the entire time. The power supply is going to serve as a bottleneck, refusing to supply more than 100 watts. 100 watts X 672 hours per month = 67.2 kwh. This isn’t some sort of crazy theory – it’s rather simple math.

    A third of that would be 23 kwh. At a cost of 10 cents per kwh here, that would save me $2.30 per month … but that’s based on the very faulty assumption that the computer is constantly using max energy. It very rarely is (few computers area), and in sleep mode uses just 1.39 watts, per the manufacturer’s specs.

    In reality, there are a lot of devices in your house that use a lot more energy than computers. The refrigerator, central air, etc. Computers get a bad rep, probably because they can do so much – but they really are not the energy hogs they appear to be.

    I suspect that you started shutting down your computer at the same time that you made some other change – curtailing use of other appliances (especially heat and AC, if they are electric), switching appliances and bulbs to a more efficient replacements, and maybe even switching to an energy plan that charges more during peak hours and less at night. I think the tail is wagging the dog in your case.


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