An energy slave is that quantity of energy (ability to do work) which, when used to construct and drive non-human infrastructure (machines, roads, power grids, fuel, draft animals, wind-driven pumps, etc.) replaces a unit of human labor (actual work). An energy slave does the work of a person, through the consumption of energy in the non-human infrastructure.
The term was first used by R. Buckminster Fuller in the caption of an illustration for the cover of the February 1940 issue of Fortune Magazine, entitled "World Energy". Alfred Ubbelohde also coined the term, apparently independently, in his 1955 book, "Man and Energy", but the term did not come to be widely used until the 1960s, and is generally credited to Fuller.
An energy slave is used to compare the productivity of a person and the energy that would be required to produce that work in the modern, oil fuelled industrial economy, although it could be applied anywhere that labor is produced with non-human sourced energy. It does not include the ancillary costs of damage to the environment or social structures. Formally, one energy slave produces one unit of human labor through the non-human tools and energy supplied by the industrial economy, and therefore 1 ES times a constant that converts to work accomplished = 1 human labor unit.
The choice to “employ” energy slaves is only at the margins of their total impact, so are called slaves because users receive the value produced by them as an entitlement of the society.
One way to look at an energy slave might be called the “macro” view. This is to look at the total flow through of energy divided by the number of persons being supported by the infrastructure where that energy is being used. It is a number that can change instantaneously as the flow through of energy changes. Although this formulation is challenged by the need to decide whom to include in the count, and the massive data-keeping it would require, it is intuitively simple because we merely divide one number by the other, and guides us in thinking about the other perspective. It is suited to large blocks of a given economy and to comparisons of economies.
Another way to look at the energy slave might be called the “micro” view. This is to look in detail at the substitution of human labor by non-human sources of productivity, in particular the modern industrial infrastructure of machines and services.
As a very simple example, ten apple pickers descend from their trees and walk to the processing shed with their apples, and then return to their trees. They have produced some number of units of work. Ten other apple pickers unload their apples into an empty box, and then return to picking. The box, now full, is carried by a field tractor to the processing shed. The work of these ten pickers plus the driver of the tractor plus all of the energy inputs have also produced that number of units of work.
The energy inputs include the life cycle share of the energy required to build and maintain that tractor and the box (called "embodied energy"), plus the fuel required to run it for the time occupied by bringing, placing, idling, and returning that box to the shed, plus the energy required to acquire, process, transport, and distribute that energy (more embedded energy).
The energy used in the two systems is not defined to be equal. The ratio of the energy used to produce an energy slave’s volume of work, through machine labor, as opposed to the energy used to produce a unit of human labor, is one of the most salient questions implicitly raised by this concept.
If we let Lw the labor of walkers, Ln the labor of non-walkers, Ld the labor of driver, Ei the non-human energy inputs, C the constant to convert units of energy into units of work. Further, let 1 Ph be one person-hour.
Then given what has been said above
And therefore, since the human labor inputs equate to the energy slave units
Supposing then that work is measured in person-hours, and supposing further that the walkers require half an hour each to go to the shed and back, that both groups take 6 minutes to fill their apple pouches, and the driver takes 30 minutes to go to and return from the shed:
The energy inputs (times the constant) replace 4.5 person-hours of labor. Returning to the original definition, an energy slave is the energy required to produce a unit of human labor otherwise than organically, so we need to convert these 4.5 person-hours into energy slaves.
In the original example, ten laborers produced their all-human work output in six hours. The other laborers plus their machines produced the same work in 1.5 hours of human labor. Therefore the energy slaves replaced 4.5/6.0 hours of human productivity, and there are 7.5 energy slaves.
The question “How many energy slaves do I have?” () is answered by looking at the amount of energy required to build and drive the infrastructure to support your lifestyle (), multiplied by slaves per unit of energy (). This would be expressed as
One of the questions that arises is:
Was the caloric expenditure of the laborers producing the person-hours less than, equal to, or greater than, the caloric energy inputs to the system? In terms of the variables used thus far, is the question if is less or equal Ei, or bigger?
In meaningful terms, did the economy use more or less energy, by using energy slaves, than it would if it had used actual human labor? This value might be a kind of benchmark, but an economy crossing this benchmark won’t notice a qualitative difference.
The implication of the energy slave unit is that each of the workers who did not walk were able to return to picking apples, and therefore increase their personal productivity. Doing the labor of 10 persons with 2.5 persons worth of work (10-7.5), the laborers with energy slaves can produce 10/2.5, or 4 times, as much work, in the same amount of time. Their personal wealth, and/or that of their employer, can be expected to increase; however, because of the huge energy investment behind the infrastructure, the margin of benefit for the employer and the workers must be less than 4 times the value of the work of the unassisted workers.
If energy slaves were actually free, then we would seek to shunt off as much labor as possible onto them. However, they are not free, and the cost of an energy slave, compared to the cost of human labor, may decide when to use an energy slave and when to use a person. A more interesting question than that about the calories used by the different systems is the question of the cost of each. The cost of human labor trends downward as the number of workers grows faster than the work available to support them, and as the number of energy slaves decreases per person. Meanwhile, as the cost of energy increases, the investment required to use energy slaves instead of people may become greater than the cost of people.
When someone discusses the amount of energy used to produce, harvest, transport and distribute a head of broccoli to a store three thousand miles away, the energy used can be expressed in terms of the number of energy slaves required to do that work. Since there are so many such deeply nested costs associated with the industrial infrastructure, we need some way to resolve the energy used to build the truck, to smelt the steel and convert petroleum into plastics, into units of human labor. Would we directly substitute an actual person walking across the country with the head of broccoli for the truck that actually carries it for the sake of comparative productivity? Or do we just divide the units of energy used by the industrial infrastructure by the number of calories used by one person to accomplish the same task, to get energy-slaves?
People who use this term want to convey in human terms the amount of energy required to support our modern American lifestyle. Another way to articulate this ratio is in terms of the energy required to grow food and transport it, as compared to the energy that food provides to a person. In a society with only human labor, you could not consume more energy than you produce in food. What does it mean when the energy required to produce food exceeds the food value it provides to a person? Just how much more energy than is contained in the food is acceptable? These are questions like those when using the "energy slave" unit, that need to be answered by people seeking understanding of these units.
In the apple pickers and tractor example there is a further consideration where the tractor is replaced by animal type of tractor power, for example a horse. If the horse is being feed on 'low-value' energy inputs such as grass grown in situ with the apple trees. The grass would have been utilized by the living ecosystem around the apple trees by, for example rotting down and providing humus but has, instead, been used by the horse with most of the energy content of the grass being used to produce waste heat by the muscular action of the horse but the remainder being returned to the apple tree ecosystem in the form of dung. Whereas the tractor has used a 'high cost' and effective non-renewable source of energy in the form of petrol, the horse has used a relatively 'low cost' fuel in the form of grass that is quickly renewed by the energy input of the sun. As long as overgrazing does not occur resulting in the degradation of the apple orchard ecosystem.