Field of Science

The Carbon Footprint of that Computer

Someone asked me over lunch yesterday if I was worried about global warming. "Worried enough to ride my bike to work through the hills of Bryn Mawr!" was my response. The conversation eventually turned to how much energy computers used - should you turn them off to save energy (and thereby reduce the amount of CO2 being dumped into the atmosphere)? Their IT support had said to leave the machines on, on the grounds that the amount of energy used to restart them outweighs any savings from turning them off at night. I thought this was not true, and some back of the envelope calculations suggest shutting down from the night (even putting the machine to sleep is not sufficient) is 10 times more energy efficient than leaving it on.

Powering up my machine takes 3 minutes at full tilt. At 120 watts, this uses up about 22 kJ of energy. If I left it in sleep mode all night (at 3.5 watts), it uses 228 kJ. I save about 200 kJ of energy, if I shut it off for the night, rather than just put it to sleep. It comes to about 44 pounds of carbon dioxide a year. It's a drop in the bucket compared to the per capita amount of carbon dioxide produced in the US (19.8 metric tons in 2003) - about 0.1%.

If you want to check your own carbon footprint, the EPA has a calculator.


  1. As an ex-chemist who is now on the IT side doing academic technology, the power issue is not all of it.

    There are a variety of arguments about the durability of various components in a computer, especially hard drives and power supplies. If the failure rates are higher if the machines are turned off and on, the carbon cost to manufacture new ones may well offset the entire power savings. I've seen arguments both ways on the durability issue.

    However, the real reason is practical. Most IT departments schedule backups, patches, updates, virus and spyware checks and such at night so that performance isn't hurt when you actually want to use the machine. This has been a constant problem in our classrooms: I had one of my workers going around on weekends making sure machines were on- otherwise they were almost useless when they woke up on Monday morning, causing many irritations for faculty who want to use them in classes.

  2. This is an excellent point, and in fact the actual advice I gave to the questioner was to ask his IT people about the night work. My institution does not do unscheduled work, for example. So things can be scheduled to be shut down at night.

    To the best of my knowledge, the durability issue is a chimera -- but you never know. I shut off at home, and since that comes from my budget, I guess I'm putting my money behind that principle.

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  4. I think the conversation eventually turned to how much energy computers used should you turn them off to save energy.

  5. What about the Carbon footprint to make all the computers to date? Has to be as high or higher then the power to use them NO?


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