It seems as if the “Power of Donuts” posting from a couple weeks back garnered a fair bit of response, and not just in the “man, I gotta get me some deep fried sugar” sort of way.

You see, the moral of the post is that leaving lights, computers and stuff on unnecessarily over the weekend is like throwing away over two-dozen donuts…but that would be in a perfect world, and one response received, from Dr. Leslie MacMillan who provides care at UHN’s Princess Margaret Cancer Centre, reminded me in a creative way (that made me laugh) that we don’t live in a perfect world, and not just in a “man I wish I could eat nothing but donuts all day” sort of way.  In Dr. MacMillan’s own words…


Leaving electrical equipment running when not in use is even more wasteful than your illustration indicates, since you have to account for the thermodynamic losses in converting donuts (or other fuel) into mechanical work and thence to electricity.

While a donut does yield 274 kcal of heat when burned in a bomb calorimeter (or oxidized in my body), the mitochondria and actin-myosin “motors” in my muscles can “capture” only about 24%* of that fuel energy and convert it to external mechanical work; the rest is dumped into the atmosphere as waste heat.  (That’s over and above my basal metabolic rate.)  (I say “only” 24% but this compares favourably with the thermodynamic efficiency of heat engines, all the more impressive considering that the body operates only a few Kelvins above ambient temperature.)

So, were I to spend the weekend pedaling my bicycle attached to a dynamo to generate the 9 kW-hr to run those idle devices, I would need to “burn” about 37.5 kW-h of donuts = 135 megajoules, or 32,300 kcal.  This is 117 donuts, nearly 10 dozen!  If I wanted to economize, I could use, as an alternative fuel, ~8 lb of pure Tenderflake lard.

Since much electricity around the world still comes from burning dinosaurs (or fossilized ferns), not donuts, I took a stab at estimating how much energy it takes to waste 9 kW-h.  Modern steam turbine plants achieve thermal efficiency of about 30 – 35%.  When you account for energy losses from heating up the transmission wires over long distances (which we minimize by using high voltage and consequently low current), the thermal efficiency of the whole process from firebox to outlet box is not vastly different from me on my bike, so let’s say 24% again.  That way we don’t have to work out another set of numbers and your 9 kW-h of electricity still needs 135 MJ worth of fuel to generate it, but fossil fuel this time. 

*Footnote:  One of those happy coincidences of Nature is that the empirically determined conversion factor from gram-calories to joules (4.18…) is close to the reciprocal of the experimentally measured fuel efficiency of human power generation (0.20 – 0.25).  We can therefore figure that (k)cal of food burned = (k)joules of external work.  This comes in handy on long bicycle tours to compare quantity of donuts eaten, mountain passes ascended, and body weight gained or lost.  For example, to pedal at 141 watts to run those devices (9 kW-h / 64 h), I need to burn (to 2 significant digits) 140 gram-cal/sec (= 500 kcal/hr).  


I couldn’t have (and didn’t) say it better myself…and while the world we live in isn’t perfect, I think Dr. MacMillan’s response is.  Thank you!