# Are humans energy efficient?

Consider how much energy it takes a person bound to a motorized wheel chair to get around. Several car batteries that maybe last a few hours and the person can’t move very fast.

Now consider a normally mobile human. A few apples and maybe a steak and they can tear around all day.

I think the ‘average’ recommended calorie intake is 1200 calories. If I remember correctly 1 calorie is the energy required to raise 1 kilogram of water 1 degree (celsius) at sea level pressure. Basically enough energy to boil 12 kilograms (20 lbs or so? or about 2.5 gallons) of water is enough to do all the things we do in a day.

Is this really efficient? Does it compare favorably with what it would take a battery to do the same work? It seems efficient. Not least because it seems like throwing a steak into your gas tank might be an interesting and cheap fuel source ;).

Are you talking about a human as a whole organsism, or on a cellular level? And, what aspect of effecient do you mean?

How you are comparing a person working after eating food to a battery using energy is also kind of confusing. The person just “recharged” after eating. The battery is dying of starvation. Also, the battery does not have the capability to seek out new charging places as a person can look for food.

Here is my question. Its a spinoff, so I’ll leave it in this thread.

Are animal/plant cellular processes more efficicient at converting fuel into energy as, say, a power plant?

Definitely a more elegant way of saying what I’m trying to ask.

Go with Wood Thrush’s question folks…it’s essentially what I was trying to ask.

BTW: As for putting a steak in your gas tank it occurred to me that would mean cars would be crapping all over the freeway. Maybe it’s not such a good idea afterall unless we fitted them with car diapers.

A human can only convert 10% of the energy it gets from food. That’s not very efficient. Im sure an auto can get more than 10% of the energy from a gallon of gas.

Well, according to “The Matrix”, humans are the most powerful energy source in the world. As I get all of my scientific knowledge from the movies, this must be correct.

Seriously, a few points:

1. Your recommended calorie intake is very low. A more accurate number is 1800-2000 calories (I think that’s the number current RDA’s are based upon)

2. This is nitpicking, but your analogy to boiling 12 kilos of water is also inaccurate. At phase transition points (solid to liquid, liquid to gas), additional energy is required.

3. A problematic part of your post is the definition of “energy-efficient.” If, as your post seems to suggest, you are measuring by how much energy it takes for humans to do things, then we are damn efficient. In this, your analogy of the motorized wheelchair is apt. Less calories (someone help me here - are we really talking about ergs, or some other measure) are used by a human to walk a block than a motorized wheelchair to transport the same weight a block. This is more an operation of musculoskeletal system, with its incredibly effective use of leverage, etc., as well as the up-to-now unreproducible operation of muscles (the ability to contract, etc.)

If, however, you are talking about the ability to convert fuel into energy, IMHO, the question is unanswerable. For example, humans are incredibly more efficient at deriving energy from a steak than automobiles, but incredibly less efficient at deriving energy from gasoline.

Now, if we put different fuels aside, then the question goes to how much of the potential energy in a given fuel is converted into useful energy. Again, there are three lines of thought that can be followed here. The first is: - how much potential energy gets wasted? (in cars as waste heat, in humans as crap) Again, unanswerable - human food has too many variations, for example, we actually expend more energy digesting celery than we derive from it.

The second is, how much energy is used to convert the fuel into usable form? Here, humans lose big time. Cars just burn the gas; humans have an incredibly extensive GI system that expends a great deal of energy extracting energy from food (chewing, peristalsis, etc.)

Finally comes the question of how efficiently is the fuel used once it’s processed? Here, I don’t know. Autos do poorly, as should by waste heat and all those (energy-packed) hydrocarbons that go out the tailpipe). Humans have mitochondria that derive energy from sugar in a three-step (sic?) process - anerobic respiration, the Krebs Cycle, and, argh, can’t think of the last (electron transport?). I just don’t know how efficient that is.

Didn’t mean to go on this long.

V.

Hmmm… If I remember correctly, cars are only about 3-5% efficient. And couldn’t a single drop of gas, if 100% efficient, transport it’s own weight 100 miles? Or something like that.

–Tim

I think the only 100% effeciency of (something) to energy would be anti-matter/matter collisions. I’m talking from brain freeze here but I think it would only take about an ounce of antimatter to put the spcae shuttle in orbit. Nuclear weapons, as powerful as they are, only manage around a 3-5% mass-to-energy conversion…maybe less. Fuel combustion (i.e. burning gasoline) is on the order of .001% efficient in converting itself to energy.

I’m almost certain on my space shuttle analogy because I remember readin it somewhere. The nuclear weapon bit I also remember reading about but I could be wrong there (although I’m sure I’m close on the numbers).

So, if Homer’s question stands, then a drop of gas that is 100% efficient could go much further than 100 miles. It could probably put a car into orbit (ok…I’m talking out my butt here but it probably would be on the order of what I suggested).

As far as SuaSponte asked I have no idea what the extra energy is that is required to pass a phase transition point. Is it a significant amount of energy to be considered (he/she said it was kind of a nitpick…just curious if it’s relevant)?

It does seem amazing that a signal from your brain that has less power than a AAA battery can make you pick up a 100+ pound object. I think I’m missing something here. A AAA-battery can’t lift 100 lbs. What gives? What am I missing (I’m obviously missing something crucial here)?

Warm blooded animals are terribly inefficent compared to cold blooded, but then again we can go lots of great places snakes only dream about.

Looking at the effieciency of a system requires one to decide where the boundaries of that system lie.

I you were to look at the efficiency of an automobile one could examine the energy conversion of fuel to kinetic energy or take it furhter to include the extraction of gas from the ground and refining it.One could look at the energy required to construct it and we could go as far as looking at the energy required in putting the infrastructure in place such as roads signs etc.

All in all very complex.

I have seen figures that state that coal burning power stations are often very mich less than 30% efficient, but then you could add in the network losses and decrease that figure further.

As an organism humans are fairly efficient I would have thought.I heard a medic on TV state that eating just one small bar of chocolate more than is required per day can cause an increase in weight of around 20stone in 5 years.
That could be a UL but it does at least show how good we are at extracting energy from food and storing it.

How effiecient are plants at converting light energy to nutrients?

Jeff_42 said:

“I think the only 100% effeciency of (something) to energy would be anti-matter/matter collisions. I’m talking from brain freeze here but I think it would only take about an ounce of antimatter to put the spcae shuttle in orbit. Nuclear weapons, as powerful as they are, only manage around a 3-5% mass-to-energy conversion…maybe less. Fuel combustion (i.e. burning gasoline) is on the order of .001% efficient in converting itself to energy.”

You’re talking about mass-to-energy conversion efficiency here, which is a rather different topic altogether!

The energy released in a chemical reaction or stored in a battery can be measured fairly precisely from its heating effect. For example, the energy released when burning 1 kg of gasoline is about 47 000 000 Joules. That is enough energy to raise the temperature of a tonne of water by 11 degrees celcius.

When we talk about “energy efficiency”, we generally mean “what proportion of the energy released can be employed to do something besides heating stuff up?” Usually for scientific purposes, we are talking about raising a weight against gravity. This is called “doing work”. In theory, those 47 000 000 joules of energy released by burning a kg of gasoline could raise a 4.8 tonne weight a height of 1000 metres.

However, if you actually got yourself a 4.8 tonne weight, and a gasoline engine and a winch and tried to haul the weight up a 1000 metre cliff, the best you could do would be about 250 metres. This is because a gasoline engine running under optimum conditions is only about 25% efficient at doing work with the energy released. The rest comes out as heat, mostly through the exhaust.
(automobiles do a LOT worse than this because the engines are throttled down almost all of the time.)

Batteries actually do quite well. If you take a fully charged lead-acid battery and discharge it through a heating element, measuring all the heat released (including the battery warming up) you find they store about 200 000 joules per kilogram.
Use the same battery and an electric motor to raise a weight, and you can get efficiencies of 75% or more if you’re careful.

As for the OP, I just don’t know. What you’d have to do is get a human to do a certain amount of work, say on a treadmill, and measure the amount of “fuel” they burnt by measuring the CO2 they breathed out while doing it. That’s one for the sports scientists. My hunch is that a battery-motor combination is more efficient, but they work in completely different ways.
Phase transition point - put a pan of water on the stove with a thermometer in it. When it reaches 100 celcius, the water doesn’t all flash to steam in an instant, right? Instead it sits there, boiling away. You have to put extra energy in it to actually turn it into steam, typically 4-5 times as much as it took to raise it to 100 celcius in the first place.
As for the AAA battery thing, the signal from your brain isn’t doing the actual work! It’s just switching your muscles on, and THEY do the work.

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Phase transition point - put a pan of water on the stove with a thermometer in it. When it reaches 100 celcius, the water doesn’t all flash to steam in an instant, right? Instead it sits there, boiling away. You have to put extra energy in it to actually turn it into steam, typically 4-5 times as much as it took to raise it to 100 celcius in the first place.

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This is known as latent heat, as opposed to sensible heat (heat changes you can feel). In this case of water, it requires 974 BTU to change 1 lb. of water at 212 F into 1 lb of steam.