Human Energy Units
Human Energy Units
Measuring Your Diet in Watts
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 Ratios
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 metric ton of TNT = 4,184,000,000 J = 1,000,000 Cal1
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”.2
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.
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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.) ↩
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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. ↩