# Comparing the heat produced by a human to that from an incandescent lightbulb

Today I was taking care of a kid and he asked me about this. As in, he said both human and light bulbs give off heat, and his teacher said a 100 watt bulb gives off more heat than a 75 watt bulb and that gives off more heat than a 40 watt bulb and so forth.

So he wanted to know what wattage light bulb gives off as much heat as your average human?

Anyone have an idea of how to go about estimating this? Short of sticking a human in an insulated box and measuring.

http://personal.cityu.edu.hk/~bsapplec/cooling.htm

Look at Sensible Heat in Table 8. Near the bottom of the page.

2400 watt hours = 2 065.0 kilocalories

1 kilocalorie is equal to one food Calorie.
2400 watt hours is the amount of energy used, or heat put out by a 100 watt light bulb.

2065 Calories is remarkably close to the recommended average daily caloric intake for adults. The vast majority of those calories end up being dumped to the environment as heat.

In terms of heat output, you can think of humans as 100 watt heaters.

Neat! So your average adult male is a 60w light bulb if he’s just sitting there, and up to a 185w bulb if he’s working out.

Good enough!
(My guess was you’d have to start with something like person X burns 2000 calories/day and that eventually ends up as heat… Of course I had absolutely no idea how you go from calories to watts.)
And after posting I see that Squink did the math. Thanks!

Yes, this correlates with a figure I saw on some Discovery channel show about the underground bomb shelters in WWII Berlin. They got very warm even in winter, because they said a human body gives off as much heat as a 100W bulb.

I always heard 100 W as a rule of thumb, too.

It’s interesting to consider how much power a person could put out. A big fellow running up stairs can certainly put out a horsepower. Some bicycling champions can put out about a horsepower for extended periods, IIRC.

And that’s why my gym has such a honkin’ huge AC system…

I guess it gives a whole new meaning if you call someone a dim-bulb. A new insult:

He’s only putting out about 40 watts if you know what I mean…

Who says today’s teachers aren’t capable of profound pedagogy?

Hm, so does this mean that if we engineered it right, we should be able to take the energy used by a lightbulb and do as much work as a human can (pulling a certain load at a particular rate, etc.)?

Yes, on average, although a human would be capable of doing more work over a short period of time. What you are talking about here is using the electricity to drive a 100W electric motor instead of a 100W light bulb. The motor will carry on delivering (nearly) 100W 24 hours a day, even when the human is resting, sleeping, etc.

I was reading in a magazine – Scientific American, maybe, or Discover – about sophisticated robotics, and a scientist was quoted as saying their goal was to get something like the human body – self-repairing, balances itself, lots of endurance, and runs on (okay, slightly more than) carrots. The same article claimed that if you stripped all the muscles off an adult human and put them into full retraction, they’d lift several tons.

I’ve found a new insult! Although, I’ll have to reference the Dope to do it…

I don’t think the exciting bit here is how much work you could get done with 100 watts, it’s how much power a 100 W light bulb uses to put out so little light.