The Physics
An encyclopedia of scientific essays

Power of a Human

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Cutnell, John D. and Johnson, Kenneth. Physics. New York Wiley, 1995: 91. "A person is making homemade ice cream. She exerts a force of magnitude 22 N on the free end of the crank handle, and this end moves in a circular path of radius 0.28 M. The force is always applied parallel to the motion of the handle. If the handle is turned once every 1.3 s, what is the average power being expended? 30 W
The Life Power Stream. Physics & Astronomy at the University of Missouri-St. Louis. This helps lift the fog put up by too many apparently unrelated energy units, clarifying for example that … a 100W light bulb (left on continuously) uses as much life's power stream as a single human being." 100 W
Micklavzina, Stanley J. Human Power and the Watt. Physics 162, Spring Term 2003. "How do you compare to a 100 watt light bulb? A human being must take in about 2500 Calories (kilocalories) of energy in his or her food each day in order to continue to function properly. This means that he or she uses up energy at a rate of (power) of 2500 kilocalories/day. Using the fact that 1 calorie = 4.2 joules, and the appropriate time conversions show that this rate of energy usage is about the same as a 100 watt light bulb." 100 W
Measurement of Human Power. Windstream Power Systems: HumanTech. "The typical average continuous power that can be generated by pedaling is about one-sixth horsepower or 125 watts, more or less, depending on the weight, strength, and endurance of the person pedaling." 125 W

When people refer to how much "power" they have, it is normal to assume they mean how much "authority" they have. But people can also have power that is measured in watts much like a light bulb or refrigerator. Power is calculated using a simple formula …

Power = Energy/Time

The average human generates around 100 watts in an average day. Depending on the person's activity, weight, and metabolism, a person's power can be slightly higher or lower. A typical American consumes about 2500 kilocalories of energy in a day. Assuming no weight gain or loss, this also means that 2500 kilocalories are used by the body in a day. With 2500 kilocalories of energy, the body is able to function properly. This allows you to do everyday ¬†activities such as reading, jogging, sleeping, etc.

Although people don't usually calculate how much power they generate, it is possible. Converting kilocalories into watts has the advantage of allowing yourself to determine how much power you are getting from your diet. The following shows a calculation of 2500 kilocalories converted into watts.

1 calorie = 4.2 joules
2500 kilocalories = 1.05 × 107 J
1 day = 86400 s
P = W/t = 1.05 × 107 J/86400 s = 121.5 W

Wei Liang Mok -- 2003