Our 100W metabolic consumption and the 10,000W one of our civilization
Updated: Sep 26, 2020
During a recent interview, Amazon’s CEO Jeff Bezos compared the average human metabolic power-consumption of about 100 watts to the 10,000 watts per-capita one of our civilization. Basically, in a given time, the energy that each one of us uses to live, is about 100 times higher than the one that our body consumes to function -- link to the video provided at the end of the post.
Those numbers were so interesting to us that we tried to figure out where those amounts come from.
For the calculation below, it is useful to recall some fundamentals; we'll do it through an example, the reader can also refer to the notes at the end of the post for some extras on energy and power. Example: if a Tesla electric car has a battery-pack of 100 kWh (energy capacity necessary to run an engine delivering a constant power of 100 kW for one hour), it takes about half a hour to charge half of it with a supercharger having a 100 kW power capacity, and it takes 1 hour to charge all of it. In reality superchargers have slightly higher power output and phenomena like electrons repelling each-others as the battery is charged up, slow down the process; however, we think we are on the same page.
Let’s go now through our calculation. The fastest part is the metabolic power consumption:
People need on average 2,000 Calories = 2 kCal/day (Calories = large calories); being 1 Cal = 4,200 joule, we have:
2 kCal = 8,400 kJ of energy per day. Trying to transform that energy into power or energy-rate, being 1 J/sec = 1 watt, we can calculate 8,400 kJ / (24 hrs x 3600 sec) = 0.09722 kJ/sec = 97.22 J/sec = 97.22 W; therefore about 100 W; first half verified! We can basically think about our bodies as 100 W light-bulbs staying on for the whole day.
Let’s go now through the per-capita consumption-rate of our civilization:
In 2017 we consumed worldwide about 157,000 TWh/yr of total primary energy supply (TPES) or primary energy. That amount includes energy harnessed from every energy sources and applied towards any industrial and technological sector - which will transform part of that into public electricity too. Accounting for the energy lost during the refinement into usable energy, the final energy consumption is about 70% of the TPES or 110,000 TWh/yr -- it includes products like lubricants that have chemical energy-content but, that are not used as fuel. Finally, the total electricity generation (then consumed after accounting for about 15% grid losses etc ...) is about 15-20% of the total TPES or 27,000 TWh/yr.
In his comparison, Bezos was referring to the developed world, therefore we will consider US & Canada as proxy (about 396 MM people combined). Since we can think about the objective of our calculation as the amount of our planet each one of us consumes, we need to start from the primary resource, that is 157,000 TWh/yr (TPES). Moreover, we need to consider only the North America share of it (US & Canada), that we found out (sources below) is about 20% of the total, about 32,000 TWh/yr. It is worth noting that the consumption of electricity is again about 15-20% of that or 5,500 TWh/yr -- about 4,500 TWh/yr for the USA and something less than 1,000 TWh for Canada. Dividing the total 32,000 TWh/yr by 396 MM people, we obtain 88,000 kWh/yr, that is equal to 241 kWh/day, that is equal to 10 kWh/hr. The last number we obtained is equal to a power consumption rate of 10 kW. The result is consistent with the 10 kW that Bezos mentioned.
Basically, rephrasing this in terms of energy from the power calculation above: in a given time-frame, in our daily activities we use as much energy as 100 people would consumes for their bodies to function. That's interesting.
Notes on energy and power:
I) Power is the energy-rate or, the energy per unit of time; it is expressed in watts = joule/sec.
II) Energy is usually expressed in joule or J = kg * (m^2) / (s^2), that is a force times a displacement. Force times displacement is work, that is indeed a very close concept of energy, being energy the capacity to do work.
III) When we are dealing with commercial matters, we usually find energy expressed in watt-hour or its multiple kWh, that is the energy resulting form the conversion of a constant 1 kW rate of work for 1 hour -- or the energy consumed by a 1 kW power engine in 1 hour.
Other useful references:
https://en.wikipedia.org/wiki/World_energy_consumption
https://en.wikipedia.org/wiki/Kilowatt_hour
Bezos' video (min 00:57:00):
https://www.youtube.com/watch?v=xv_vkA0jsyo&t=3622s
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