Average energy of a 42-gallon barrel of oil is 5.8 million Btu, or British thermal units. By definition, one Btu of energy will raise 1 pound of water by 1 degree Fahrenheit, so about 75 Btu are needed to boil a cup of water to make your morning coffee or tea.

The 5.8 million Btu figure is called a Barrel of Oil Equivalent, or BOE. One barrel of oil has the same energy content as 5,800 cubic feet of natural gas. A cubic foot of natural gas contains about 1,000 Btu. For electricity, 1 barrel is 1,700 kilowatt hours.

So, is oil really worth $100 a barrel? Another way of looking at it is to compare oil to a horse. A horse laboring a standard 40-hour work week (eight hours a day, five days a week, 50 weeks a year) would have to labor for more than one year to produce the energy in a barrel of oil. Do you think a horse could be fed and maintained for a year for $100? Not likely.

Human labor is even worse. A fit human adult can sustain about one-tenth of a horsepower, so a human would have to labor more than 10 years to equal a barrel of oil.

Btw it also shortest explanation for quality of life changes that happened in 20th century.
As well as many newest wars for resources. And, of course, why robots are much more effective.

P.S. Btw slightly different calculations exist, some say about 12-13 years of human work.