1 Barrel of oil = 1 Year of hard human labor.A barrel of oil contains 42 gallons. It can be made into about 20 gallons of gasoline (give or take, depending on the grade of the oil). Each gallon of gasoline contains about 36 kilowatt-hours of chemical energy (kilowatt-hours are the number of kilowatts, a measure of power, times the number of hours, yielding a measure of energy). An efficient internal combustion engine turns about one quarter of that energy into useful work, with the rest lost as heat. One horsepower is equivalent to about 3/4 of a kilowatt. However, one human working hard continuously can only put out about 1/10 to 1/5 of a kilowatt (compare the power output of a human to a one-horsepower horse). A recent article in Bicycling on the Tour de France showed that the average power output (during the several hours per day of the race) of a top-finishing bike racer, Floyd Landis, was 0.23 kilowatts, or about 1/3 horsepower, continuous. In a 75 minute time trial, the same cyclist was able to put out 0.38 kilowatts continuously -- a full 1/2 horsepower.
The 20 gallons of gasoline made from one barrel of oil contains about 180 useful kilowatt-hours. If we divide that by say, 1/8 of a kilowatt -- a generous continuous output for a fit person -- we get 1440 hours of hard human work. Let's assume that a person can put out this 1/8 of a kilowatt for 6 hours per day. That is, half of the output of a top Tour de France cyclist for a continuous 6 hours (not counting breaks) per day. This means that you would need 240 days to get 180 kilowatt-hours (or more, if you are a dimmer bulb), which is minimally equivalent to one year of 5-days-a-week very hard labor by a fit human. This boils down conveniently to: ONE BARREL of oil = ONE YEAR of hard human labor.
This calculation makes sense when you compare digging a foundation or grading a road by hand, Roman empire style, to doing the same thing with an oil-powered bulldozer, roadgrader, and backhoe. A barrel of oil currently costs $57. I think this price *waaaay* underestimates its true worth to us humans. Even in the poorest third world country, you currently have to pay a human more than that to do hard labor for a year. In the US, the minium legal wage is over $10,000 a year.