Energy Consumption

Computer in Sleep Mode vs. SUV


In a column "It's the Server, Not the Computer," in The Kings County Advertiser (a weekly newspaper published in Kentville, Nova Scotia), Feb. 26th, 2008, Wendy Elliott wrote (in response to an earlier comment about comparing a computer's electricity usage in the sleep mode to that of an SUV): "A typical desktop (computer) will use 1-6 watts in sleep mode.  I'm not sure how that compares to using an SUV environment-wise."

When I read this, my immediate reaction was that it is high time someone put a few numbers into this debate.  After all, the numbers for this are simple, and would quickly settle what seems to be an endless morass of uninformed speculation.

For starters, let's look at the fuel consumption of an SUV.  There are dozens of SUV models for sale by several manufacturers, and they vary quite widely in their fuel consumption.  For a reasonably representative average, let's take the fuel consumption of an SUV to be 15 litres per 100km (about 19 miles per Canadian gallon).

The next step is the energy content of gasoline.  This varies somehwat depending on what kind of gasoline, but a reasonably representative average is about 35 megajoules per litre.

When we multiply 35 megajoules per litre by 15 litres per 100km, we get 525 megajoules per 100km.  That is, 5.25 megajoules per kilometre is a good average figure for the energy consumption of a typical SUV.

Now we turn our attention to the desktop computer in sleep mode.  You suggested "1-6 watts" and this agrees with my information.  Let's take the highest figure, 6 watts (the preferred assumption if one wants to make the computer look as bad as possible).

We are focusing our attention on the energy consumption of a computer in sleep mode.  Let's assume our computer is operated in sleep mode continuously for a whole year.  Drawing power at the rate of 6 watts, in one year our computer will consume 53kWh (kilowatt hours), or 190 megajoules.

Now we are able to make a meaningful comparison between the energy consumption of a desktop computer in sleep mode and an SUV.

Dividing 190 megajoules by 5.25 megajoules per kilometre, we get 36 kilometres.

A desktop computer operated in sleep mode 24/7 for a whole year will consume as much energy as an SUV driven a distance of 36km.  Please remember that we took the most adverse assumption; if our computer consumes less than 6 watts in sleep mode, the comparable driving distance of our SUV will be less than 36km in a year.

In ordinary circumstance, a computer does not spend 24 hours every day in sleep mode.  A reasonable estimate, of the time spent in sleep mode by an average desktop computer, will be at most in the range of 4000 to 5000 hours a year.  Let's take the high end of this estimate (the preferred assumption if we want to make the computer look as bad as possible).  The 36km then should be divided by 8760 hours and multiplied by 5000 hours, which works out to 21 kilometres.

Conclusion: Over a year, an ordinary computer in sleep mode will consume as much energy as an SUV driven 21km.  That's one trip Wolfville to Kentville and back.

Now we know how a computer is sleep mode "compares to using an SUV environment-wise."

The claim that "their carbon footprints are similar over a year" is obvious nonsense.

Please feel free to have these numbers checked by anyone you like.  All I ask is that you take make an effort to ensure that whoever you consult is competent in such matters.

If you, or your consultant, prefer to use your own average figures for SUV fuel consumption or gasoline energy content, or computer power consumption, of course you are free to do so, making the appropriate adjustments in the calculations, but such adjustments, if done responsibly, won't change the 21km calculated above by any amount worth mentioning.


Ivan Smith
Canning Wolfville, Nova Scotia

PS: Note that our computer in sleep mode will operate for 1600 hours on one dollar's worth of electricity (at today's residential electric rate in Nova Scotia).  Some people recommend that we turn our computers off instead of allowing them to run in sleep mode.  If our computer is operated in sleep mode for 12 hours each day, the energy saved by turning it off will amount to about $2.60 a year (if you ignore the big picture, which means the true saving is closer to eighty cents in a whole year).  The suggestion that Horton High School "would save over $2,000 per year... if computers weren't left on stand-by" is pure fantasy.  I can't do any numbers on this because I don't know how many computers the school has, but my educated guess is less than $200, certainly nowhere near $2,000.






Here's a suggestion that could make a good high school science project.

Measure the power consumption of an ordinary desktop computer.

You need a watt-hour meter.  You measure the energy consumption over a
day, or (even better) over a week or two.  Then divide the energy consumed
by the time, and that's the power consumption.  It's not complicated.  Include
just the computer and its monitor.  Exclude the printer, scanner, and other
peripherals.  In a school, you could connect together a cluster of five or
ten computers fed through one meter, to get a better reading.

You might think (as I did until a few days ago) that this information can easily be found
just by doing a search on the WWW.  Not so.  Try it for yourself.  You'll get lots of hits,
but you will find more bafflegab than solid information, and much of what you do find
will be years out of date.  You'll look a long time before you find useful information that
applies to the computer you have at home.



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First uploaded to the WWW:  2008 March 10
Latest update:  2012 October 18