Understanding large numbers
Posted by Pete McBreen 16 May 2010 at 12:52
While reading Eaarth I came across an number that needed further thought. Supposedly a barrel of oil contains as much energy as a man working can put out in 11 years.
OK, so that needs some cross checking. A barrel of oil is 42 US gallons, or 158.987 liters (used to link to a Thailand website that was near the top of the google search for that conversion, now linking to wikipedia). That barrel of oil has about 6.1 GJ or 1,700 kWh, or 38 MJ per liter. roughly 10.5 kWh.
Maximum sustained energy that a human can put out is around 100W over an 8 hour period, so 0.8kWh/day, but that would be classed as extremely hard labor, so a better number to use would be about half that, so a good estimate would be 0.5kWh/day. Allowing for 250 working days/year, that means a strong, fit human can put out about 125kWh/year.
Comparing this to the 1,700kWh per barrel of oil, a human capable of a sustained output of 50W would take about 13.6 years to generate that much energy, so the figure of 11 years energy per barrel is reasonable. Other numbers in Eaarth suggest that in North America, the annual consumption of oil per person is about 60 barrels, so in a year, a human uses the equivalent of 660 humans worth of work.
Since that is hard to imagine, a smaller scale to look at it is that 1 liter of oil, enough to drive a car for maybe 15 km or 10 miles (assuming an efficient car), uses as much energy as an average human could put out in a month of working 8 hours a day, 5 days a week. (21 days work at 0.5kWh/day for the 10.5kWh in a liter of oil).
Small wonder then that the thoughts of Peak Oil and Global Warming from excess CO2 have everyone concerned in one way or another…