Watts For Lunch? (Or Why Humans Are Like Light Bulbs) : Krulwich Wonders... Take a bunch of broccoli, or make it a Slurpee, burger, pizza and fries, swallow, and ask yourself, "How much energy did I just consume?" Enough to light a flashlight? Run an electric toothbrush? If I were a lunch-eating light bulb, how long would I glow? Here's the answer.
NPR logo Watts For Lunch? (Or Why Humans Are Like Light Bulbs)

Watts For Lunch? (Or Why Humans Are Like Light Bulbs)

There's a new lunch place down the block, so like you do when the menu looks interesting, I walked in and ordered something mysterious, which for me was the "Red Lentil and Edamame Salad," mostly because I can never remember what edamame is, and because that word suggests doing something slightly frightening, like munching accidentally on one's mother.

How Much Energy Am I Eating? Enough To Power A Flashlight?

What arrived was a bowl of lentils, roasted carrots, raisins, mint and (I'm guessing) edamame beans. I took the bowl to a window seat, and that's when my mind began to wander. My mind doesn't need much to go free. It slips off whenever I let it, when I'm by myself and alone with my thoughts, which, at this moment, were: "So I'm chewing these beans and I'm breaking them into little bits, which will become littler bits in my stomach, (bond-breaking, as the chemists would say) so I'm turning food into energy. But how much energy am I getting? Does a salad produce enough calories or watts or whatever, to light a flashlight? Or run an electric toothbrush for 10 minutes?

(Do you ever do this? I do this all the time.)

Light bulb
Robert Krulwich/NPR

After lunch I looked up the answer. I found it in a fine little book by Wayne State Professor Peter Hoffman, called Life's Ratchet: How Molecular Machines Extract Order from Chaos. In it, he says humans will typically eat roughly 2,500 calories a day.

1.5 Million Joules Is ... ?

Professor Hoffman is a physicist. He goes on to make some quick calculations. If one food calorie equals 4,184 joules of energy, at 2,500 calories a day, that means our bodies break down or release 1.5 million joules. Sounds like a lot, no? But if you divide those joules by the number of seconds in a day (86,400), that works out to a rate (where 1 watt = 1 joule per second) of about — 120 watts a day. In other words, that's all I need to dream, wake, dress, shower, work, walk to a restaurant, order a salad, ask myself how much energy am I using, and then look it up, think about it, and write this essay. I can power all trillion cells (of me) for a day at the same rate that it takes to light one 120-watt light bulb.

That's it?

That's it. Peter Hoffman writes, "Humans talk, write, walk and love using the same amount of energy per second as a light bulb."

I'm humbled. I will now confess that when I got back from the salad place, I diddled, I called friends, yakked with office mates and used up lots of time to avoid writing this, and yet — down deep, at a cellular level, it turns out I'm a mind-boggling display of energy efficiency. You too, of course.

On certain afternoons, this is a nice thing to know.