A single barrel of crude oil (costing ~$50 to extract, ~$60 market price) contains about 5.7 million BTU or ~1,700 kWh of work potential, equivalent to about 11 years of full-time human labor; handicapped ~40% for machine inefficiency, roughly 4-5 years of human work is replaced per barrel-plus-machine, so the ~100 billion barrels of oil equivalent humanity burns annually is like adding a 500-billion-person workforce to a real workforce of ~5 billion.