I was curious so I estimated. If we take daily intake at 8000 BTUs (~2000 calories), then we get 0.015Q BTUs in 1800 (US population: 5M[0]), 0.22Q BTUs in 1900 (US population: 76M), and 0.82Q BTUs in 2000 (US population: 223M).

That's only humans. Factoring in all working animals, maybe we assume some multiple of that. 2x? 10x? 100x seems too much.

In the end, I'm not convinced it would be significant enough to warrant inclusion - even if only because US population was so relatively small.

This does lead me to wonder what the graph looks like for energy consumption per capita.

[0] https://en.wikipedia.org/wiki/Demographic_history_of_the_Uni...

Wood at the beginning of the period covered by chart is at 0.2 Q BTUs, so, using the 10x factor, food would be on the same order of magnitude.

It's also missing the amount of sunlight Earth gets from the Sun that's not used as solar energy. Also the energy that plants receive as nutrients from soil and air. It's also missing the energy that a mother transfers to her child in the form of love.

Food as an energy source is actually an important aspect. Things like domesticated animals represent significant sources of energy, on the order of thousands of calories per day. More nutritious crops also confer a significant boost to available energy. This had significant impact on the disparate levels of development between Eurasia and the Americas before the early modern period.

"The Measure of Civilization" by Ian Morris is a book I'd recommend on this topic.

And there was a paper about how improved energy access was key in nomadic to sedentary evolution. People didn't get better, they found ways to get more energy by farming.

I assume the chart is only counting energy important for human economy (and not whatever it is that you're bringing up). Food as energy source was crucial part of the energy mix before fossil fuels - food-powered animals were tilling fields, hauling goods, powering machinery etc. (not to mention less savory aspects of it like slavery - slaves were essentially food-powered machines for picking cotton, harvesting sugar cane etc.). Nowadays majority of that work is done by machines powered by fossil fuels, and food is used mostly for us to sustain ourselves (i.e. prevent starvation) and not as an economically-productive resource.

I suspect food was the biggest energy source in the economy, above wood and renewables (i.e. watermills and windmills powering various machineries and pumps).

> also missing the energy that a mother transfers to her child in the form of love

probably simpler analysis to define a boundary around human population and only estimate energy flows that cross that boundary, ignoring internal flows