Also, that is the net energy expenditure of the body, but not the input required. We use a relatively antiquated form of the internal combustion engine to turn

Also, that is the net energy expenditure of the body, but not the input required. We use a relatively antiquated form of the internal combustion engine to turn food into energy, which seems to be around 25% efficient, at best. It's also built on chains of other fairly inefficient biological entities, so it's hard to say what the overall efficiency is, but suffice to say that we're prevented from using the most efficient energy sources.