Energy Units

Biologists are concerned with energy flows over a wide range of scales, from a single bacterium to the whole earth. Here are some useful conversion factors and some tables of data illustrating biological energy problems.

Units of Force

Units of Pressure

Units of Energy & Work

Units of Power

Energy for Bacterial Growth


 % Dry


per Cell

per Second

 ATPs used
per Second

Energy for


 2 billion

































These figures were calculated by Albert L. Lehninger for the E. coli bacterium, which has a division time of 20 minutes. Every 20 minutes during growth the cell must make all of the components needed for a new cell.The biosynthesis requires 2,400,000 molecules of ATP and 400,000 molecules of oxygen every second. Note that almost 90% of the ATP energy goes into making new proteins. From: Albert Lehninger. Biochemistry: the Molecular Basis of Cell Structure and Function. NY: Worth Publishing, 1975.

Energy for Hummingbird Migration

Some ruby-throated hummingbirds do non-stop migration flight across the Gulf of Mexico for 800 kilometers (about 500 miles). The flight takes 10 hours, a speed of 80 km/hr (50 mph). It is amazing because the bird weighs only 3 to 4 grams (a little more than a penny). This weight must include both the flying machine and the fuel.

During flight the birds use about 250 ml of O2 per hour. The energy required can be calculated from the oxygen consumption:

Power = (0.25 liters O2/hr)(4.82 kcal/liter O2) = 1.2 kcal/hr

Total energy required = (1.2 kcal/hr)(10 hrs) = 12 kcal

The amount of fuel required can be calculated by assuming that the hummingbird stores the energy as fat at 9 kcal/gm:

Fat required = (12 kcal)/(9 kcal/gm) = 1.3 gm

If the energy were stored as carbohydrates (glycogen) 3.0 gm would be needed, plus some extra water that glycogen carries with it. About 3 times more fuel weight would be required. This is why migrating animals store energy as fat instead of carbohydrates. Hummingbird data is from: Oliver Pearson. The metabolism of hummingbirds. Scientific American, January, 1953, p. 69-72.

Energy Stores of the Human Body

 Storage Form


 Energy Stored

 Time of Use
min at 3 mph

 Miles at
3 mph

ATP & Creatine

 ATP = 0.1
CP = 0.15














In addition to these molecules the body can burn protein for energy. There is 10-15 kg of protein in the body, giving a potential of another 40,000 to 60,000 calories. Much of the protein is required for cell structure and function, however, so it is not clear how much protein is available for energy. Data from:

Peter Hochachka & George Somero. Biochemical Adaptation. Princeton University Press, 1984, chapter 4.
George Brooks & Thomas Fahey. Fundamentals of Human Performance. NY: Macmillan, 1987, chapter 1.

Power Output for Human Walking and Running



















































Values from 0 to 4 mph are for walking. For 5 mph and above figures are for running. Calculations are for a 60 kg person.This person will use 1440 kcal each day at rest. If she walks 2 miles/day this will add 94 kcal to her energy use. If she were to run the 2 miles she would add 188 kcal (running costs twice as much as walking in energy). Figures are calculated from equations in: American College of Sports Medicine. Guidelines for Exercise Testing and Prescription, 4th edition. Philadelphia: Lea & Febiger, 1991, p. 285-300.

Power Costs of Human Activities

This table gives a picture of relative energy costs of common activities. A MET is 0.0169 kcal per min per kilogram. To calculate your energy output multiply the MET figures by 0.169 and your weight in kilograms. Suppose y

 Energy Cost

 Sleep, watching TV while lying


 Reclining or sitting while talking, writing, reading, kissing

 1 to 2

 Standing quietly


 Light home activities: cooking, dish washing, watering lawn

 1.5 to 2.5

 Office work

 1.5 to 2.5

 Driving car


 Playing music

  2 to 3

 Light carpentry, plumbing, electrical work


 Walking 3 mph


 Bicycling, leisurely

 4 to 6

 Painting, remodeling

 4.5 to 5

 Playing baseball



 5 to7

 Carrying groceries, boxes, furniture

 6 to 8

 Backpacking, cross country skiing

 7 to 9

 Playing basketball, football

 8 to 9

 Digging ditches, carrying bricks

 8 to 9

 Running 6 mph


 Fast rope jumping


 Running, 8 mph


 Running, 10 mph


ou are backpacking (8 METs) and weight 70 kg:

Power = (0.0169 kcal/min-kg)(8)(70 kg) = 9.46 kcal/min = 660 watts

These figures are from: Barbara Ainsworth, William Haskell, Arthur Leon, David Jacobs, Jr., Henry Montoye, James Sallis & Ralph Paffenbarger, Jr. Compendium of physical activities: classification of energy costs of human physical activities. Medicine and Science in Sports and Exercise 25: 71-80, 1993.

I question 2 of the figures from the Ainsworth table. Sexual activity ("active, vigorous effort") is given only a 1.5 MET rating, while showering is given a value of 4.0 METs! I wonder if they mixed up the 2 values.

Human Power Limits





 Energy Used
Power X Time

 Single vigorous jump or lift

 < 1 sec





 20 sec




 Mile run

 5 min





 2 hr




 Manual labor

 10 hr





 24 hr




The maximum power the body can produce is seen in jumping or in rapidly lifting heavy weights. We can sustain this level of activity for a second or less ("the harder one works the sooner one must stop" Bent). If we cut down the power output we can go for a longer period of time. Power output of the body is determined by the amounts of different energy stores and by the enzymes that release the energy. ATP and creatine phosphate can be broken down very rapidly (high power), but the total amount is small. Burning of fats for energy is very slow (low power), but there are huge amounts.

The table comes from: Henry A. Bent. Energy and exercise. I: How much work can a person do? Journal of Chemical Education 55: 456-458, 1978.

Use of Energy in Human Society

In addition to metabolism of food we use energy, mostly in the form of fossil fuels derived from living creatures, for industry and transportation. Use of energy varies widely from country to country. In the US we use about 42 barrels of oil equivalent per capita every year, while in India, Nigeria and the Phillipines the amount is 2 to 3 barrels per capita per year.

Using the conversion factors from above (a barrel of oil is equivalent to 6.1 million joules) it is easy to convert the 42 barrels/year into watts or calories/day:

(42 barrels/yr)(6.1 billion joules/barrel)/(365 days/yr) = 702 million joules/day

(702 million j/day)/(4.18 j/cal)(1000 cal/kcal) = 168,000 kcal/day = 8140 watts

A fairly good estimate for the average power output of a human is 100 watts. Comparing this with the figure for industrial energy use we see that industrial user is about 80 X the metabolic energy use. In a sense each of us has 80 energy servants working for him continuously. For further information on this subject see the September 1990 special issue of Scientific American (Energy for Planet Earth).

Energy Budget of the Earth



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