What is the total energy that has been derived from fossil fuels over the past century?

Answer 1

A barrel of Oil contains about 6,000,000 brithish thermal units.
Oil usage is measured in Billions of barrels per year.
In 1925 usage reached about 1 Billion Barrels.
By 1950 usage reached about 3 Billion Barrels.
10 Billion Barrels was reached in the early 1960's
20 Billion Barrels was reached in the early 1970's
25 Billion Barrels was reached about the turn of the century.
Making some guesses at values from the chart on oil supply from the DOE, I estimate that the the total energy from oil over the last century to be about 6*10^18 BTU's