Part of the reason is that humans are not run on batteries. Humans are more similar to fuel cells in that we consume oxygen to produce energy. We inhale oxygen, combine it with our energy stores and exhale 80% of the mass[2].

An average American man needs about 3400 Calories each day which corresponds to about 1 kg of oxygen consumed[1]. Similarly the average American woman uses 2550 Calories and .75 kg of oxygen.

[1] Design Rules for Life Support Systems http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2004001... [2] http://www.livescience.com/49157-how-fat-is-lost-body.html

3400 calories? That seems awful high. Perhaps if you are into bodybuilding but no way an average person needs that much. Closer to 2000-2500 maybe.

Mostly the latter, I believe.

The human brain uses the incredible 20 Watts to achieve things that modern megawatt supercomputers aren't capable of.

I think the figure is closer to 100 W average across the day since the brain cannot exist in isolation of the body which needs ~2000 calories daily, but still, we're fairly efficient. We're built with advanced nanotechnology.