Not all energy sources are created equal in terms of energy return on investment — the amount of energy they produce for the amount spent to produce them.
These bars represent the estimated maximum energy return on investment of both renewables and fossil fuels. Oil's ratio has declined over time with the increased difficulty of bringing it to consumers. Note its EROI value in 1930, 1970, 2000, and 2005.
SOURCE: CHARLES A. S. HALL AND JOHN W. DAY JR./AMERICAN SCIENTIST, GRAPH: MATTHEW T. STALLBAUMER
The energy return on investment for firewood includes what you spend cutting and splitting it. However, after some rest and a good meal it's 100 percent renewable!