# Does the average person run on 96.85 watts?

1. Mar 15, 2006

### wasteofo2

I tried to figure out how many watts a person runs on who eats a 2,000 calorie diet.

First, dietary calories are really kilocalories, so someone eating a 2,000 calorie diet is really eating 2,000,000 calories.

1 calorie is 4.184 joules, so multiply 2 million by 4.184 and you get 8,368,000 joules.

Divide that by seconds in a day, 86,400, and you get 96.85.

Is that right? Does the average person run on 96.85 watts?

2. Mar 15, 2006

### Pengwuino

Well people aren't 100% efficient.... but yah that's about right.

Pretty amazing to think that we use as much energy as a lightbulb or two.... but a lightbulb is nothing to what your average house runs on. It's like, 10-40 people would have to be eating food and working 24/7 to produce the energy a household needs. Our house uses like 2-4kwh/h.... wait... how does that work. 2 kilowatt-hours/hour..... yah that makes sense....

Last edited: Mar 15, 2006
3. Mar 16, 2006

### Staff: Mentor

Yeah, that doesn't include what you pee and poop out, but you're in the right order of magnitude. 70w is typical for the HVAC industry, barring more specific info about what they are doing (ie, a weight room).

4. Mar 27, 2006

### quark

It is still worse. The maximum human efficiency is 20% during bicycling and generally as low as 5%.

5. Mar 27, 2006

### chroot

Staff Emeritus
Efficiency in what terms? In terms of motive energy?

Keep in mind the body's primary use of energy is to maintain temperature. Unlike an internal combustion engine, the heat is not simply waste -- it's essential to the survival of proteins and enzymes necessary for human life.

- Warren

6. Mar 27, 2006

### Staff: Mentor

Besides - in the sense we're talking about (generating heat), humans are near 100% efficient. The energy that doesn't go toward building new cells (thus, absorbed as chemical energy) all becomes heat.

7. Mar 27, 2006

### Clausius2

I think there's another way: use Stefan-Boltzmann law:

$$q=\sigma(T_{body}^4-T_{room}^4)A_{body}$$

That's a lose by radiation (which is not negligible at all under standard room conditions). Try to plug in numbers, I remember it works.

8. Mar 27, 2006

### moose

If you are running up a flight of stairs or something, you could be using more than 1000 watts, if you are sprinting that is....

9. Mar 28, 2006

### topsquark

Yeah, and go ahead and figure out how many flights you have to run to use up just one can of Pepsi! (I was astonished anyway!)

-Dan

10. Mar 28, 2006

### chroot

Staff Emeritus
I can't understand why anyone would want to run up stairs to get exercise. I can't even think of a less enjoyable place to exercise than a dank, confined indoor stairwell. I can burn off a can of Pepsi in 3 miles on my bike -- three very enjoyable miles surrounded by nature and fresh air -- in nine minutes.

Big deal.

- Warren

11. Mar 28, 2006

### Clausius2

For running 3 miles in 9 minutes the path must be facing downwards all the time. When I was in Spain, 17-18 years old (my best moment of shape), I used to run 10 km in 15 minutes in a open-air closed road circuit with my road bicycle. :tongue2:

12. Mar 28, 2006

### chroot

Staff Emeritus
What? I'm not talking about running, I'm talking about bicycling. And 3 miles in 9 minutes is 20 miles an hour, which is my normal cruising speed on flats. My fastest recorded flat sprint is about 32 mph for about five minutes.

- Warren

13. Mar 28, 2006

### Ouabache

Burning calories surrounded by nature sounds very appealing!! Yet I do understand why folks want to run up stairs.. It builds up some different muscles, especially those that come in handy for hiking mountain trails and surface swim with scuba gear.. It is also an intense workout to build stamina. Though I would save it for the colder months or bad weather. We run hills in fresh air, during nice weather for the same effect. :tongue2:

14. Mar 28, 2006

### Staff: Mentor

Clearly, you did not wrestle in high school. "Enjoyment" has nothing to do with why you run a stairwell (though usually "punishment" did...).

15. Mar 29, 2006

### quark

That is what I meant. If we difine the efficiency as useful energy/energy input, the human actions have a maximum efficiency of 20%. The metabolic activities consume much less energy, for example, human heart consumes about 5 Watt/min.

Including human body, inefficiency of any machine finally leads to heat whether it is required or not. If all the heat generated by the body is required to maintain the temperature, there won't be any perspiration, perhaps.

16. Mar 29, 2006

### chroot

Staff Emeritus
I don't know what a Watt per minute is.

Besides, like I said, the efficiency of muscle is much higher than 20%, even though the entire human body has to keep itself warm to function, and thus uses a lot of energy to maintain temperature.

Except when you're burning a thousand calories an hour on a bike. Even if your muscles are 90% efficient, you're still dissipating as much heat from muscle contractions as you do while sleeping.

- Warren

17. Mar 30, 2006

### quark

ah.. that is a blunder. Please read it as Joules/min. For other things, I should better refrain myself from commenting.