Myself, Coding, Ranting, and Madness

The Consciousness Stream Continues…

One Night to Breathe With You, One Night of CO2

15 Apr 2011 13:40 Tags: None
Leaving a computer on overnight creates enough CO2 to fill a double-decker bus

At least, that's what the poster proclaimed. It shouldn't have, I have since realised, having found a similar one on council website which had the very important phrase 'for a year' in it, but that was all after I'd written this post.

Now, i doubt normally give much thought to what's on posters on my way to work, but this did give me pause. As a computing person, I regularly have multiple machines on for a large amount of time. I even have my own always on server (Eliza, for those of you who remember my machines). So, I did begin to wonder how much CO2 I might be pumping into the atmosphere

Of course, that number by itself would be entirely worthless, so I set out to find a some good comparison points first. The first of these was the human body, which was thankfully easy to get well reasoned figures for thanks to This post of annual emmision of CO2 by humans. The two separate calculations gave rather different answers (~0.5kg and ~0.9kg), so I'll be taking the lower bound of 0.5kg per person per day

Now, the next easiest fact to find out was the volume of a double decker bus - taking the good old faithful London Routemasters (or, more accurately, the smallest cube that the original 27'6" model could fit in) gives a volume of 89.6m31. Now, like those of you who know your chemistry, you'll know that 1 mol of a gas at room temperature2 and pressure will have a volume of 24dm3, or 0.024m3. So, to fill a double decker bus with a gas will require approximately 37303 moles of gas

The molar mass of carbon dioxide, if we assume only the most common isotopes are present, is 44 g / mol4, giving the mass of CO2 required to fill a double decker bus to be around 165kg5. So, the breath of 330 people for an entire day is required to fill up a bus. If my computer, left on over night, produces the same amount, then there really is going to be a problem with the world6, as having Eliza on all day is using up the CO2 quota for most for a village7.

So. now we come to the question of how much carbon dioxide a (my) computer produces. In and of itself, the answer is 'a negligible amount', unless it catches fire for some reason. But the real issue here is the amount used in producing the electricity that the computer uses to run. Now, for these calculations, I'm not going to be using my server, but rather my laptop, as it's some what easier to actually find out how much power is being drawn. If my mains ammeter turns up at some point, I may redo this for the server (or, I could turn off the fridge and freezer and test it from the incoming meter :P). The conversion from power to carbon comes from the official UK course, Defra (The Department for Environment, Food, and Rural Affairs) and their Guidelines for Company Reporting on Greenhouse Gas Emissions Annexes8, which gives the conversion factor to be 0.43 kg / kWH, meaning that a generating a kilowatt hour of electricity produces the same amount of carbon dioxide as one person breathing for a day. If we take the 'night' to mean outside of standard offices hours (say, 9 to 5), then the machine would be active, but unused, for sixteen hours. My laptop, which will draw less than most desktop machines, has a one amp fuse in the socket9, meaning that the maximum power of the system is 230W10. Now, disregarding that fact that it will automatically power down the screen, hard drive, and eventually the entire thing, and assuming it's drawing all available power for the entire time, this is still only 3.7KWhrs11, around one one-hundredth of the 330 people or kilowatt hours needed to fill a bus12.

However, when you take the 'for a year' into account, these figures begin to add up. I've been very generous with the power usage of this laptop (the adapter is actually rated at 95W, and should be about 80% efficient; of that, at most 60W will be in use when the battery is already fully charged13). 1KWh a day, which just about makes up the required amount is actually only 60W - the amount of power drawn by a single old-style lightbulb

So, there we have it: Leaving my laptop on overnight isn't really that much worse for the environment than having a kid14 or being able to see during my all-nighters. Oh, and for reference: To generate a full double decker bus of carbon dioxide (165kg) from national grid electricity in 16 hours requires the power usage of 24KW15, which is the same as leaving you kettle forcibly stuck on for the entire night. So, no all night tea parties!

Congratulations to anyone who knows what song was in my head when I wrote the title for this post. Hint: José González

  1. 1 ↑ 8.38 * 9.14 * 4.38 = 89.558736, 89.6 to 3sf. I'll skip the error calculations
  2. 2 ↑ I'm taken room temperature to be about 296K to make the volume figure round. Actual room temperature as defines in science is 298.16K (25 Celcius) which is much hotter than I like my rooms anyway
  3. 3 ↑ 89.6 / 0.024 = 3733.33333, 3730m3 (3sf)
  4. 4 ↑ Carbon-12 is exactly 12g / mol by virtue of definition, and Oxygen-16 is as close to 16 grams as makes no difference at 3 significant figures. 12 + 16 + 16 = 44
  5. 5 ↑ 44 * 3730 = 164120g = 165 +- 5 kg (hopefully over-estimated error bars)
  6. 6 ↑ I'm not a climate change sceptic, don't get me wrong. I just like running the numbers first. For example, I don't drive, take aeroplanes or, tbh, use public transport except on long trips. I keep my lights off, and I re-use stuff where I can. And, yes, I still call them aeroplanes.
  7. 7 ↑ Making the assumption that the nature is actually completely unable to adapt to our changes, and that total CO2 intake isn't increasing with availability.
  8. 8 ↑ This linked version dates from 2005, and makes reference thatit was due to be reviewed in 2010. I am, however, unable to find an updated version
  9. 9 ↑ Originally 3, but it's also designed to run at 110V, and the rating on the devices says 1.5A, so it's all good
  10. 10 ↑ Mains has officially been 230V, not 240V in UK/Europe for some time. Not that anyone really noticed, as it was more of a bookkeeping thing, making it so that people hit their targets (or so I understand it)
  11. 11 ↑ 230 watts * 16 hours = 3680 watt-hours
  12. 12 ↑ Actually going back to the mass makes a double decker bus be 165 / 0.43 = 383.721 kilowatt-hours. 3.7 / 384 = 0.00964 or 0.96%
  13. 13 ↑ I know this because I also have a 60W adapter which doesn't charge the battery but runs the computer
  14. 14 ↑ One of my computers shares the tendency to wake up loudly during the middle of the night too...
  15. 15 ↑ 383.721 / 16 = 23.98