How Much Power Do Humans Really Emit?

[WarPhalange]

I've heard this figure thrown around a bit and I'm having trouble wrapping my mind around it. The idea is that a human just sitting around like I am now gives off about 100W of power.

I'm eating a pack of fruit snacks now, worth about 190 Calories. So, a quick calculation...

4.12J/s = 1 cal/s; 100J/s ~= 25cal/s; 1000 cal = 1 Cal => 1 Cal/40s; Under a minute to burn a Calorie. A day has over 1200 minutes, so you end up burning let's say 1800 Calories.

Is this where the "2000 Calorie Diet" that you see on food packaging comes from? They just assume you're a lazy *** and don't move at all? Or is 100W just way too simplified?

 

[CRGreathouse]

Or is 100W just way too simplified?

Yep.

 

[russ_watters]

Yes, 100W is oversimplified, but it is also a bit high. Average is more like 70W. Remember, not all of what you burn ends up as heat - some provides energy for other chemical reactions.

 

[Gokul43201]

Somewhere between 60W and 100W is common for low to medium activity levels.

In some chip fabrication cleanrooms, each person entering the cleanroom switches off a lightbulb while entering (and switches it back on when leaving). This special set of lightbulbs does not serve any illumination purpose, but simply helps to minimize fluctuations in the cleanroom temperature.

 

[mgb_phys]

Remember that food calories are really Kcal, ie 1Kcal = 4200J

 

[russ_watters]

I looked for it online and couldn't find it, but here are the values ASHRAE uses (in btu, sensible and latent, with watt converstions):

Seated at theater: 225S, 105L = 66W, 31W
Moderately active office work: 250S, 200L = 73W, 59W
Walking, standing: 250S, 250L = 73W, 73W
Light bench work: 275S, 275 L = 81W, 81W
Moderate dancing: 305S, 545L = 89W, 159W
Heavy work: 580S, 870L = 170W, 255W
Athletics: 710S, 1090L = 208W, 319W

Obviously, my 70W figure was sensible heat only. Evaporation from sweat and breathing adds a lot to the total heat dissipation, so it may be better to say the total heat generated by a person nearly at rest is more like 130W.

 

[noagname]

is this counting the brain or not

 

[WarPhalange]

Remember that food calories are really Kcal, ie 1Kcal = 4200J

I remembered to make that distinction...

 

[mgb_phys]

I remembered to make that distinction...

I know, I reread your calcs after the edit window, sorry.

 

[DaveC426913]

Somewhere between 60W and 100W is common for low to medium activity levels.

In some chip fabrication cleanrooms, each person entering the cleanroom switches off a lightbulb while entering (and switches it back on when leaving). This special set of lightbulbs does not serve any illumination purpose, but simply helps to minimize fluctuations in the cleanroom temperature.

Wow.

 

[waht]

That's interesting. So there is about 600 billion watts of power being wasted by the Earth's population.