One can contrast the energy content in humans with the external energy sources we have conquered, to give us an idea how important the development of external energy sources are. Let us do a back-of-the-envelope-calculation. A human needs around 2,500 calories per day (2,9kWh). We use around 80 percent of that just to stay awake, think, eat, sleep, breath etc. So a bit less than 600 Wh is perhaps for work. Hard working people could perhaps develop 800 Wh, but they will also eat more. On a yearly basis the average person consume around a million calories, which correspond to 1.16 MWh. The average American uses 7.71 tons of oil equivalents (toes) of energy (the average Brit 3.8, the Swede 5.65 and the Senegalese 0.25), which corresponds to 90 MWh, i.e. the energy in the annual food consumption for 77 persons, or if we calculate with the twenty percent that we actually use for work it would correspond to roughly 380 persons. On the other hand there are also energy losses for the conversion of external energy to useful work. If we assume that it is forty percent we land at 150 persons.
So depending on how we count, each American has somewhere between 75 and 400 “energy slaves” working for him or her, and the richer ones have thousands. The global average energy use is of course lower, and represents figures between 18 and 90, which translated to the total population would mean somewhere between 120 and 600 billion energy slaves. In that perspective it is certainly no surprise that we have taken over this place! Another way of looking at it, and also to put in an economic perspective, is that a barrel of oil represents the energy of 25,000 hours of human toil, i.e. 14 persons working a year with normal Western labour standards. The cost for pumping the oil is not more than a few dollars per barrel, but even with an oil price of many hundred dollars per barrel, it is very cheap even compared to human slave labour. The beauty of the oil slaves is that we don’t have to feed them at all. The slaves of the past, even under huge oppression and extortion took most of the feed for themselves, just to survive. With this perspective, our current wealth is easily understood and de-mystified. Even hundreds of years ago, the person that had hundreds of others working solely for him, could lead a comfortable life, long before industrial society and capitalism.
 1kWh = 3.18 MJ = 860 kcal.
 a toe is a common unit for energy and express the amount of energy released when a ton of oil is burnt. 1 toe = 42 GJ = 11 MWh = 10 Gcal.
(I have posted something similar one and a half year ago, but these figures are updated)