I thought this was an interesting passage from an article by Rudy Baum in C&E News titled "Sustainable Growth Is An Oxymoron"  in which he highlights a point made by Bill McKibben:

The fact is that, eventually, we have to learn to live off the sun in real time. That’s not going to be easy. Fossil fuels aren’t just fossilized sunshine; they’re concentrated fossilized sunshine. As McKibben points out in “Eaarth,” 1 barrel of oil yields as much energy as 25,000 hours of human manual labor—more than a decade of human labor per barrel. The average American uses 25 bbl per year (some estimates are quite a bit higher), which, he writes, is like finding 300 years of free labor annually.