Check your numbers
I have looked at all of your videos and find some to be good, some to be blather, and some that need a bit of correcting. Since I am part of the Transition Movement here in NW Washington state, I feel compelled to comment on a couple of glaring errors in Rob Hopkins’s video. Since I have put together a sustainable model of agriculture that uses mostly human labor and very small amounts of fossil fuel for tillers, I feel it is important to get the numbers right. Here is my comment, which I have also posted on my Local Harvest blog (look under F.A. Farm on the blog page) and also on my own blog. http://fullattention.blogspot.com/. By the way, I viewed the videos through the Energy Bulletin website and I am a Nation subscriber.
In the Rob Hopkins interview, which is part 13 of The Nation’s video series, he makes a couple of glaring errors. These two errors are at 21:00 minutes into the video and immediately after. Here is the weblink: http://www.energybulletin.net/media/2011-04-13/how-climate-change-puts-g… The first error is one he has made consistently in his videos over the three years that I have been viewing them. As he has done before, he states that the energy in a liter of petrol is equivalent to thirty-five days of hard human manual labor. This is more or less a throwaway line to make his point, but he overstates the calories in human labor by almost a factor of four. This is significant if we really want to get traction with economists and engineers who actually measure this kind of thing. There is a lot of potential energy in a liter of gasoline or petrol and we can indeed measure them and compare them to human energy by using calories or joules or BTUs or kilowatt hours or horsepower, all of which can be translated one into the other. (Caveat: By calories I mean kilocalories. It is well understood in human nutrition information that we are really talking about kilocalories and sometimes we see it written as “calories.” This is just a blip in semantics.)
Unfortunately, Hopkins does not state how many hours of human labor per day, so let’s plug in eight hours. He also doesn’t give an energy equivalent for human labor per hour, so I will use my own metric. I am a farmer and do hard physical labor every day, but I don’t consume more than other people of my size, which is normalized at 2,500 calories per day for a human male. I also don’t sweat copiously all day long (it is not like playing football/soccer for ninety minutes, for example) and the physical exertion has to be maintained over a long day, sometimes sixteen hours in the summer. Thus it is reasonable to apportion my labor output at what it takes to get me through sixteen hours. Since we use about 500 calories in an eight-hour sleep period, that leaves 2,000 calories spread out over sixteen hours, or 125 calories per hour. In my experience, this is a valid measurement that is robust (i.e., you can throw all kinds of bad data at it and still get valid results).
Now if we calculate Hopkins’ statement with our own energy and time values we can see that he is saying a liter of petrol is equivalent to 35,000 calories (35 x 8 x 125 = 35,000). Now, checking this out on the web, we find that a gallon of diesel does indeed have an energy value of 35,000 calories per US gallon (you may have to convert from joules or BTU’s, depending on the website). So, if Hopkins meant to say that a gallon of diesel has an energy value of thirty-five days of human labor, he would be spot on. (The corresponding value for gasoline is 31,000 calories per gallon, by the way.) However, Hopkins says that a liter has this energy value, and since there are 3.785 liters in a US gallon, he is exaggerating by a factor of 3.785 or nearly 4. Perhaps he should be saying, “A US gallon of petrol has the energy value of thirty-five days of human manual labor.” Alternatively, he could say, “A liter of petrol has the energy value of over nine days of human manual labor.” He doesn’t even need to say “hard human manual labor.” Every audience gets the point about manual labor, since most people will do almost anything to get out of it.
Which brings me to my second point. A bit after his throway line about the energy value of a liter of petrol, he says that a liter can power a car for thirty miles. Since there are 3.785 liters in a gallon, this would mean that a British car gets over 113 miles per gallon. I doubt there are any cars in Britain with this mileage, much less on average, which is implied in his statement.
Rob Hopkins is fighting the good fight, but he needs to polish up his numbers. It really does matter, especially if we want to calculate our own energy consumption or compare the efficient human engine to the inefficient internal combustion engine. The comparison is stark. We don’t need to engage in hyperbole.
Apr 13 2011 - 3:10pm