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Human Energy Units

Posted: Jul 5th, 2022 - Modified: May 20th, 2023

We eat food to get energy, which we measure in terms of dietary Calories.

We use electricity to provide energy to our homes, which we measure in kilowatt-hours.

Physicists often measure energy in joules, which is the amount of energy needed to apply a newton of force across one meter of distance.

These different units of “energy” aren’t merely analogous; they’re all measuring the same thing. It’s totally sensible to talk about the Calories burned by your toaster, or to say that your bowl of pasta contains a kilowatt-hour of energy.

Energy Conversion


We can use the following ratios to convert between various measures of energy:

  • 1 dietary Calorie (Cal) = 4184 J ≈ 0.001162 kWh
  • 1 kilowatt-hour (kWh) = 3,600,000 J ≈ 860.4 Cal
  • 1 BTU ≈ 1055 J ≈ 0.252 Cal
  • 1 metric ton of TNT = 4,184,000,000 J = 1,000,000 Cal1

Energy Comparisons

First of all, how expensive would your food bill be if you could eat electricity?

The “standard” adult needs 2000 dietary Calories per day,2 which converts to about 2.3 kWh (or 8.4 megajoules or 7900 BTU). Let’s call this amount a “Day of Food Energy” or DoFE for short.

At 16 cents per kWh3, electricity costs only 37 cents per DoFE. Pretty cheap! Michael Phelps, in 2008, claimed to eat a whopping 10,000 dietary Calories per day, which would still be less than two dollars worth of electricity. Alas, man cannot live on voltage alone.

Gasoline would be an even cheaper source of food if you could eat it. (You cannot. Do not try. You will die.) A gallon of gas contains a bit over 15 DoFE.5 At /$3.62 per gallon,6 that works out to less than 25 cents per DoFE.

A single gram of uranium fuel produces over 172 DoFE of electricity in a typical reactor,7 and up to 9500 DoFE (26 years of food energy) could be theoretically produced. A single DoFE could be provided by a speck of Uranium the size of a grain of sand (in a reactor; do not eat uranium).

One gram of antimatter, reacting with one gram of matter, would release over 20 million DoFE.8 (Honestly, that’s less than I expected. Sci-fi has made me overestimate the power of antimatter.) If only people could eat neutrinos and gamma rays, then a mere pound of antimatter could feed the whole world.

Comparisons to explosions are the easiest to make, since a “ton of TNT”, as a unit of energy, is exactly equal to 500 DoFE (one million Cal). A megaton blast is equal to 500 million DoFE, which is roughly the amount of food energy consumed by Americans each day.9

Lazily pulling some examples from the wikipedia page on TNT equivalent:

  • Mount Saint Helens: 24 Megatons days of American food consumption
  • Tsar Bomba: 55 Megatons days of American food consumption
  • Krakatoa: 200 Megatons days of American food consumption
  • All nukes ever detonated, combined: 540 Megatons days of American food consumption

(I know I sound like a broken record here, but please don’t try to eat antimatter or atomic bombs,… or uh, volcanic eruptions.)

Power usage

  • A watt (W) is 1 joule per second.
  • The “standard” adult needs 2000 dietary Calories per day.
  • There are 86400 seconds in a day.

Combine these conversions to find that 2000 Cal per day is about the same as 97 watts. I’ll round that up to 100 watts, and call it 1 “Manpower”.10

We can then conceptualize the power use of appliances in terms of person-equivalents.

Appliance ManPower
Microwave Oven 6-13
Electric Kettle (US) 15
Space Heater 15
Gaming PC 5

Note that these are comparisons of the rate of power usage. Not comparisons of the daily energy usage. Most people don’t leave their tea kettles running 24 hours a day.

  1. The energy of explosives is often measured in “(metric) tons of TNT”. While the energy actually released by TNT is quite variable, the unit is defined to be equal to 4,184,000,000 joules. This means that a “gram of TNT” is equal to 4184 joules, exactly one dietary Calorie. By contrast, a gram of vegetable oil contains about 9 Cals. (The thing that makes explosives dangerous isn’t that they contain a lot of energy; it’s that they release that energy very very quickly.) 

  2. Your dietary energy use will vary based on a number of factors. The USDA has a calculator. I’m tall and a bit chunky, so my energy needs are closer to 3000 dietary Calories. 

  3. Price varies by market. 16 cents per kWh is the average cost of residential electricity in the US in February 2023

  4. In February, 2023, the average price of 1 pound of all purpose white flour in US cities was 55 cents. Looking at the nutrient label on the flour in my cupboard, a pound of such flour contains 1650 dietary Calories. By coincidence, these numbers work out to exactly 3000 dietary Calories per dollar, or 2/3 of a dollar per DoFE 

  5. According to this page from the EIA, a gallon of finished motor gasoline with 10 percent ethanol contains 120 thousand BTU. 

  6. Average Price of Gasoline, All types, US City Average: 3.622 dollars in February 2023. 

  7. One metric ton of low-enriched uranium generates 400 million kWh of electricity in a light water reactor. So 1 gram produces 400 kWh ≈ 340 thousand Cal ≈ 172 DoFE. But light water reactors only extract around 5 percent of the energy from the fuel. According to this page, a gram of Uranium can release 79 thousand megajoules ≈ 9500 DoFE 

  8. As per Wolfram Alpha, 2 grams of matter contains 1.80e14 Joules of relativistic energy ≈ 4.30e10 Cal ≈ 2.15e7 DoFE. Similarly, 2 pounds of matter contain ≈ 9.75 billion DoFE. 

  9. 334.8 million people times 3000 dietary Calories (1.5 DoFE) each: That gives 502.2 DoFE, essentially one megaton of TNT equivalent. Is 3000 Cal a reasonable estimate? This page from Our World in Data plots data showing the US consumes 3800 Cal per person each day, but that’s “consumption” in the economic sense. This paper mentions an estimate of 2895 Cal intake per day for adults in the years 1999-2000, but says food waste makes this very difficult to measure. My guess is that 3000 Cal is an overestimate, but not an absurd one. 

  10. It’s the amount of power needed to fuel a man. Contrast with “Horsepower”, which is the power that a horse can apply while tugging on a rope to lift things. Note also: this webpage claims that a typical untrained person can exert around 100 watts of power via pedalling. So the idea of calling 100 watts “1 ManPower” works on multiple levels. 

Comments or questions about this page? Please send a message to RobertMartinWinslow at gmail dot com. Feedback is welcomed.