# Thread: How much heat does the human body produce?

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## How much heat does the human body produce?

I've an idea for a product. Before I can even think of starting the design work though, I need to know how much heat energy the human body puts out. This would be at a time when the body is at or very near a 'rest' condition.

I have found a few numbers but for the life of me, I cannot get something 'solid' and even more frustrating are the units involved.

Calories, watts, ergs, BTU's and others, have all assaulted my poor calculator in an attempt to something I can wrap my head around.

Now, BTU's are probably the preferred units, as these are the units the device I have in mind, is normally talked about in. Yes, I'm being a tiny bit foggy on 'the device' in mind, sorry about that.

So my two questions are:

1. How much heat energy (or whatever term is appropriate) does the human body, at rest, produce?

2. If not given in BTU's, is there a source for conversion of such heat units, preferably into several other unit types?

Reading this, it is obvious I'm not very conversant with any of the terms, but I am trying to learn!

2. Try looking at ASHRAE's web site, here. For conversion, try bustling over to the NIST's website.

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At rest people generate about 4 BTU a minute in the developed world. That's about 70 watts.

4. Originally Posted by Ronald Brak
At rest people generate about 4 BTU a minute in the developed world. That's about 70 watts.
I'm curious as to your reasoning, Ronald. Treating the human body as a 310 K blackbody with a surface area of 2 m2, I get about one kilowatt. That's definitely back-of-the-envelope, and probably a bit too big, but it's an order of magnitude larger than your value.

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A person burning 8,000 kilojoules a day burns about 5.5 kilojoules a minute. About 70% of kilojoules are burned at a person's at rest basal metabolic rate, which mostly ends up as heat, which would make it about 3.9 kilojoules a minute. We very rarely actually burn that little energy, so bump it up to about 4 kilojoules a minute which is almost the same as 4 BTU.

6. Originally Posted by Tobin Dax
I'm curious as to your reasoning, Ronald. Treating the human body as a 310 K blackbody with a surface area of 2 m2, I get about one kilowatt. That's definitely back-of-the-envelope, and probably a bit too big, but it's an order of magnitude larger than your value.
You're forgetting radiation influx from the surroundings which are at 290-295K, treat it as a blackbody of the same area and subtract from your original number.
And remember that the skin temperature is less than the core so it's only 306K.

With skin temperature at 306K and surroundings at 295K, I get 135W for a naked person of 2 m2 which is upper range of skin area, only a factor two off, easily explained by clothes to further reduce the actual temperature of the radiating surface.
Last edited by HenrikOlsen; 2009-Nov-27 at 04:51 AM.

7. Originally Posted by HenrikOlsen
You're forgetting radiation influx from the surroundings which are at 290-295K, treat it as a blackbody of the same area and subtract from your original number.
And remember that the skin temperature is less than the core so it's only 306K.
I figured it was the difference between influx and outflux, I just didn't want to do that calculation.

Thanks to you both for the replies.

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Since most of the time we aren't changing weight very rapidly, and most of us aren't converting very much of our food into mechanical energy, we must surely output an amount very similar to the amount we eat.

If 2800 Cal is a typical daily amount to eat, and 4200 is the conversion factor from Cals to Joules, then the output in Watts is
2800*4200/(24*60*60) = 136W.

Gratifyingly similar to Henrik's number. Actually I suspect many of us eat more than 2800 Cal, but at the same time the conversion is less than 100% efficient.

9. Digging into my memories of very-long-ago thermodynamics classes, when we discussed air conditioning, I remember something about the latent heat of the water vapor released by a person being about as important as their heat output, per se.

ASHRAE, or comparable international organizations (ASHRAE == American Society of Heating, Refrigeration, and Air conditioning Engineers) would have the best numbers.

10. Originally Posted by Ivan Viehoff
Since most of the time we aren't changing weight very rapidly, and most of us aren't converting very much of our food into mechanical energy, we must surely output an amount very similar to the amount we eat.

If 2800 Cal is a typical daily amount to eat, and 4200 is the conversion factor from Cals to Joules, then the output in Watts is
2800*4200/(24*60*60) = 136W.

Gratifyingly similar to Henrik's number. Actually I suspect many of us eat more than 2800 Cal, but at the same time the conversion is less than 100% efficient.
That would assume that we are able to extract 100% of the energy contained in our food i.e. that poo has no calorific content. This is very much not true for human beings, in fact more so than for many other animals. I tried to establish a realistic percentage in a thread I started a few months ago (the link escapes me now) but nobody was able to give me a definitive answer.

clop

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## basal matabolic rate

I think I've seen ~ 70 watts for a resting human being several times. If you begin to starve right now, you disappear at about the same rate as a wax candle and generate heat pretty equivalent to it. That produces a weight loss of ~ a pound a day. People who say they can't lose weight are eating regularly. If you exercise, work hard, and think a lot, you lose it faster. If you sit still, you lose it slowest. That's seven pounds a week, but starvation is not good for the physiology, because of ketosis. Eat a little less, be more active, even reading helps.

pete

SEE:http://en.wikipedia.org/wiki/Basal_metabolic_rate

There was a federal grant a few decades ago, to explore heating schools with.....children.

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That would assume we are able to extract 100% of the energy contained in our food.
The kilojoule figures put on foods generally represents the amount of energy humans are usually able to extract from them. So if you drank lemonade, little or no energy in that would end up in poo, it would all be used to power or fatten you. But if you ate lettuce, a lot of energy in the form of (almost completely) indigestable cellulose would end up in poo, but this energy isn't counted, so we take in more energy than is written on the packageing our food comes in, but we generally can't use it.
Bateria in our bowls can use some of it, which creates a grey (brown?) area as far as the amount of energy we use goes.
Last edited by Ronald Brak; 2009-Nov-28 at 03:40 AM.

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Originally Posted by Ripcord
1. How much heat energy (or whatever term is appropriate) does the human body, at rest, produce?
Between 1,800 and 3,200 kcal a day, depending on a lot of factors (gender, fitness, size, musculature).

2. If not given in BTU's, is there a source for conversion of such heat units, preferably into several other unit types?
Here's that in BTU's (thermochemical): 7,143 to 12,699 BTU

Source for energy conversion.

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Originally Posted by Ronald Brak
The kilojoule figures put on foods generally represents the amount of energy humans are usually able to extract from them.
I thought they were largely obtained from bomb calorimeter experiments by combusting them entirely in oxygen. Which is why I said that conversion in the body is not 100% efficient. But the efficiency of conversion depends on what you eat. Conversion of stuff like refined sugars is very efficient. Dietary fibre, on the other hand, is very inefficiently digested, and can actually reduce the efficiency with which other stuff is digested.

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Originally Posted by Ivan Viehoff
I thought they were largely obtained from bomb calorimeter experiments by combusting them entirely in oxygen. Which is why I said that conversion in the body is not 100% efficient. But the efficiency of conversion depends on what you eat. Conversion of stuff like refined sugars is very efficient. Dietary fibre, on the other hand, is very inefficiently digested, and can actually reduce the efficiency with which other stuff is digested.
These days caloric content found on nutrition labels is calculated from the amount of carbohydrate, protein, and fat content of the food rather than bomb calorimetry. See here, here and here, for example.

Nick

16. Originally Posted by Ripcord
...How much heat energy (or whatever term is appropriate) does the human body, at rest, produce?...If not given in BTU's, is there a source for conversion of such heat units, preferably into several other unit types?
LIVING IN SPACE by G. Harry Stine:

"During sleep, the body's energy expenditure is about 65 Calories (kCal) per hour. At rest lying down, it rises to 80 kCal/hr and, when sitting up, to 100 kCal/hr. During light exercise, this jumps to as much as 200 kCal/hr and, during heavy physical work, to 500 kCal/hr."

Note Google does conversions directly from the query box. Just use the IN keyword.

Query: 65 kcal per hour in watts
Result: 65 (kilocalories per hour) = 75.5444444 watts

Query: 65 kcal per hour in btu per hour
Result: 65 (kcal per hour) = 257.768344 btu per hour

Query: 500 kcal per hour in watts
Result: 500 (kilocalories per hour) = 581.111111 watts

Query: 500 kcal per hour in btu per hour
Result: 500 (kcal per hour) = 1 982.83342 btu per hour