How many watts does the human body use

  1. how many watts does the human body use

    i guess 500w
     
  2. jcsd
  3. selfAdjoint

    selfAdjoint 8,147
    Staff Emeritus
    Gold Member

    I once read, in connection with crew in a space habitat, that an adult human radiates like a 100 watt light bulb. You could figure the other from that, assuming eqquilibrium. Your guess looks to be in the ball park.
     
  4. jim mcnamara

    jim mcnamara 1,500
    Science Advisor
    Gold Member

    You can work it out from knowing kCal of food metabolized that results in equilibrium - it's called basal metabolic rate (BMR). It's the rate of oxygen consumption (or CO2 production) per Kg of body weight when the bodyy is at rest.

    play with this applet:
    http://www.bmi-calculator.net/bmr-calculator/
     
  5. selfAdjoint

    selfAdjoint 8,147
    Staff Emeritus
    Gold Member

    Hey thanks for that. I got my BMR to be 1554 (ignoring the decimals) which is a number I have been trying tofind.
     
  6. My BMR is 1888, so what's that mean my wattage is?
     
  7. jim mcnamara

    jim mcnamara 1,500
    Science Advisor
    Gold Member

    Watts measure power, Kcal measures energy. If you're willing to go with Watt-hours to get everything in the same units of measure:

    1 KCal = 1.163 watt-hour, if I remember correctly.

    So, 1888 * 1.163 = 2915 Watt-hours
     
  8. The recommended UK Male Calorie intake is 2550 Calories/ Day
    1 Calorie (Nutritional) = 4186.8 Joules

    So assuming zero weight gain, and waste having no nutritional value you must dissipate 123.6 J/s or Watts, or the heat generated by burning 12.5oz of coal.

    What the proportion of heat/motion is depends on how warm your personality is....
     
  9. marcusl

    marcusl 2,120
    Science Advisor
    Gold Member

    BMR has units of power. The web site given in the link above is using kcal/day, although units aren't given there, so to convert to watts
    1888 kcal/day * 4.18 kJ/kcal * 1 day / 8.64e4 sec = 91.3 Watts

    BMR depends on body weight and percentage body fat, since muscles generate heat, age, sex, and other factors including individual variability.
     
  10. Most of the energy is consumed by the brain I think .
    :smile:
     
  11. Nice replies fellas.

    So ~100W seems to be a good ballpark figure for a human body's power output. Funny that a 100W light bulb is on par with the human body!

    SelfAdjoint says 'he heard' the brain uses 25% of this, so 25W, I wonder how true this is.

    Very interesting thread so far, what else can we think of that's on topic?
     
  12. selfAdjoint

    selfAdjoint 8,147
    Staff Emeritus
    Gold Member

    So what you could do now is reexpress your calories of food (which are really kilocalories, you know) as watt-hours, and see what your energy balance is. Or different kinds of excercise, what are their wattage requirements? Getting away from the dietician's calorie seems to be a refreshing way to think about our daily regimen.
     
  13. marcusl

    marcusl 2,120
    Science Advisor
    Gold Member

    Years ago I visited the precision metrology lab at HP Labs, which was temperature controlled to keep the instruments, meter bars and such constant. At the entrance to the room was a panel of light switches that controlled 100W bulbs inside. Each person entering turned off 100W, and turned it back on when they left, so the heat load inside stayed approximately constant.
     
  14. DaveC426913

    DaveC426913 16,239
    Gold Member

    The brain uses 25% of the body's oxygen, so that seems about right. Unless there are anaerobic prcoesses at work in the body?
     

  15. I couldn't agree with you more self. Calories and kilojoules are like pints and quarts to me. I much prefer terms like eV and Watts since batteries and machines and such all label their power usages in these terms. It makes comparisons easy and much more interesting.

    @Marcus
    Now there's a real-life application that really solidifies and roots this thread in the practical. Its very interesting to hear your example.
     
  16. Look around and imagine how we remain.

    We do not radiate all the energy we consume, so if we radiate like a 100 Watt light bulb we actually require a higher wattage since the human body is a very very ordered thing indeed. Just look at your eye ball in the mirror and try to imagine all the meticulous planning and processes that are occurring, the structure it possess. Now think how is all this kept ordered? Energy of course, energy input is required. So eating food is needed to keep our high entropy state going the way its going. To recap in a line: energy inputed is outputted through various radiations and is used to maintain order.
     
    Last edited: Nov 28, 2006
  17. marcusl

    marcusl 2,120
    Science Advisor
    Gold Member

    Careful about mixing up energy and power units. In fact, the energy content of food is ubiquitously quoted in Calories, so there isn't much getting away from it. That's why treadmills and exercise bikes display the energy you have burned in units of Calories instead of joules--you can directly compare input and output.

    Another commonly used unit is Mets (metabolic equivalents). 1 met is the same as your BMR; you expend about 4 mets mowing the lawn, while 12 mets is about as hard as you can exercise for any length of time. It's a useful gauge of relative effort.

    Thanks!

    Energy balance is conceptually pretty simple: whatever doesn't come out as mechanical work is dissipated as heat. There is some entropy content to thoughts that might be calculated by Shannon's theorem but I think it's completely negligible compared to the rest.
     
  18. Ya I know calories/joules are energy. Did you see I mentioned eV? Calories = joules = eV, you just need to use simple conversion formulas. Watts is power, which is simply energy/time. I'm not confusing anything here.

    btw, I don't know what country you are in, but most nutritional info in Canada is in both kCal and kJ, so its not as ubiquitous as you think.
     
  19. selfAdjoint

    selfAdjoint 8,147
    Staff Emeritus
    Gold Member

    I'm with li'l bro. If we go to Joules and Watts, people might even start to understand energy vs. power. The idea of using the same units in your regimen as you see on your electric bill might be really liberating. And I like mets too. If you could get mets ratings for different activities the way you can find the calorie rating of almost any food, you could plan so much better.
     
  20. marcusl

    marcusl 2,120
    Science Advisor
    Gold Member

    That's cool! In the US food packaging gives only Calories.

    Charts are readily available online. See, for instance,
    http://www.hsph.harvard.edu/nutritionsource/Exercise.htm
     

  21. 500 watts = 7.17 food calories (kilocalories) per minute
    http://www.google.com/search?hs=Xf7...=500+watts+to+Calories+per+minute&btnG=Search

    The average person needs between 1000-2000 food calories per day.
    500 watts is about the amount of power an average person can put into the cranks of a bicycle for a few seconds. World class cyclists can deliver 500 watts to the cranks for more than 30 minutes and burn 4,000-8,000 food calories in a race.

    Incidentally, 500 watts into the cranks of a standard racing bicycle will push it to a speed of about 30 mph on flat ground.
     
    Last edited: Nov 29, 2006
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?