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# Calories into Watts6

## Calories into Watts

(OP)
If a bicycle rider burns 300 calories in 30 minutes, how much power output in Watts are we talking about?

### RE: Calories into Watts

5
300 calories=1255 J. 30 minutes=1800 s, so power= 1255/1800, or 0.7W

You probably meant kilocalories, in which case multiply everything by 1000. The efficiency of the human body is often quoted as 40% so that's a power output of 280W, which is not unreasonable according to

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Calories into Watts

(OP)
Woah, thank you so much...! This will really be helpful.

### RE: Calories into Watts

I can vouch for that as well.
Though 280W is a bit high from my experience personally. Not calculation wise. (calc's look good)
I base this off of my gym's equipment; if I do 30min at 180W it will give me about 300kcal. Of course, these machines are largely inaccurate.
I guess, I am burning less energy :P

### RE: Calories into Watts

Some more background/order of magnitude numbers:

An average (comfortable) human body puts out about 100W of heat at rest (through convection from skin and lung surfaces), a number my school physics teacher used to pull out of the air when it was a cold morning, but the classroom was full.

If you apply that to a 24 hour period, you get: 100 x 3600 x 24 = 8640000 [J]

Convert to kcal: 864000/4184 = 2065 [kcal]

Which is roughly the number of "calories" we are told by dietitians that we need to eat per day. This is before wasting energy by doing external work (like riding bikes up hills/into wind). Putting in less than this will result in internal energy stores being used up, putting in more will add to internal storage. However, this trivial 1st law analysis is rarely given the time of day by anyone outside of the engineering world. Most normal people are 1st law deniers.

I like to point out to the dieting brigade that we are simply 100W candles, with body fat as the fuel. A pound of body fat stores about 3500 kcal of energy, so a hibernating human would be losing weight at a rate of about 1/2 a pound a day. This is the practical limit for a diet that doesn't also include doing physical work.

Steve

### RE: Calories into Watts

Don't assume that calorie burn rate will become 100% useful power output. If anything, humans are quite inefficient, generating lots of waste heat.

If you are interested in human mechanical power output (judging from pasts posts, you are), then calorie burn rates are superfluous information. You just want raw mechanical power output, e.g. torque x RPM on a crank.

### RE: Calories into Watts

Yeah hence the 40% efficiency number in my calc. However reading wiki they suggest that is high

The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolic cost, as can be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of generating ATP from food energy, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overall efficiency of 20 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, one manufacturer of rowing equipment calibrates its rowing ergometer to count burned calories as equal to four times the actual mechanical work, plus 300 kcal per hour,[15] this amounts to about 20 percent efficiency at 250 watts of mechanical output.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Calories into Watts

Have any of you done the high-school science lesson of running all the kids up a flight of stairs?
The teacher puts them on a scale to record their weight first, makes a show of hanging a measuring tape down the stairwell, then times each kid's trip up the stairs with a stopwatch.
Then all the students take their data back to class and work it out. One kid in my class got about 700 Watts. I won't say what I got!

STF

### RE: Calories into Watts

I like the chart in Greg's first post. I think the "First Class Athletes" line is conservative eg:
1. Eddie Merckx sits nearly 20% above the line.
2. If you infer Eddie's shorter-term output as parallel to the line you end up at about 1200 Watts for 0.1 minute
3. Eddie was an endurance athlete. I believe sprint cyclists can generate up to 2,200 W for a short time.
4. 2,200 using a bicycle translates to about 3000 W (4 hp) with an "optimum mechanism" - impressive!

je suis charlie

### RE: Calories into Watts

I did a little research on this for my electric cars and energy efficiency presentation for my kid's gr 7 class. I took 100 W for an 8 hour day as a decent figure for a sustainable mechanical work output, or one "human power". A "horsepower" is about 740 W for the same day, so about 7.5 people-power. One man manipulating a four-horse team was a tremendous increase in power- in the control sense rather than the energy sense. The figures I saw put thermodynamic efficiencies (work output per unit food energy input) for humans and horses in the ~ 15% range. Muscles aren't heat engines, but it's an interesting comparison.

### RE: Calories into Watts

Somptinguy:

"I like to point out to the dieting brigade that we are simply 100W candles, with body fat as the fuel. A pound of body fat stores about 3500 kcal of energy, so a hibernating human would be losing weight at a rate of about 1/2 a pound a day. This is the practical limit for a diet that doesn't also include doing physical work."

It's a lovely hypothesis, but it doesn't work that way.
If your body needs energy, the first thing that will be burned is glycose from the blood and glycogen from the liver. After that, glycose will be synthesized from protein by depleting muscular tissue. Only after this has reached an unsafe level will the human organism start burning fat. This does not start until after 12...24 hours (sometimes even longer), which explains why calorie-reduced diets don't work (except in your hibernation scenario, which is not really practical for humans

Benta.

### RE: Calories into Watts

The numbers that I refer to are from Allen & Coggan Race Category Table
It lists output in W/kg of body mass
For elite riders
full time 6-6.5
1 min 11-12
5 sec 23-24

The current estimate (they won't release actual power meter data) for the current 1 hr record (54.5km) takes about 675W for the entire hour.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Calories into Watts

Benta: bologna. Where'd you hear all that stuff anyway? That sounds like the pitch from the latest fad diet and/or supplement MLM scheme .. virtually all of them ... "the other diets didn't work because ... whereas we do that one thing differently!" Even a calorie-reduced diet based solely on eating bologna WILL work. In fact, nearly all effective diets are based on calorie restriction in one form or another. Some of them happen to cater better to the behavior and cravings of modern humans and thereby are practiced for a longer time.

Also, when the body needs energy, it clearly goes after multiple paths at once, and which ones are burned preferentially will depend on your degree of fitness and the type/intensity of activity undertaken. Looking at an extreme, if you're starving to death, clearly you do not run out of muscle before your body begins to burn fat - the fat disappears long before you lose the ability to walk or pump blood. Distance running doesn’t break down muscle as fuel. To get to that level of catabolic activity, you’ll need to combine a diet almost entirely void of protein with a high mileage, high intensity running schedule. It does happen to some runners, and if they continue they'll start peeing red and end up in the hospital. During endurance exercise a well-trained athlete will metabolize lipids ... both intramuscular ones and belly fat (adipose in general). There may even be some benefit to consuming lipids during endurance exercise .. and from experience I can tell you that it certainly feels good to eat a cheeseburger with bacon at mile 37 of a longer run ...

### RE: Calories into Watts

I went to the internet to find support for the above ... here are some key results:
http://fitstar.com/happens-muscles-exercise-2/

The Choice of Fuel Depends on Exercise Intensity
As you can see in the graph below, as the intensity of exercise increases, the dependence on carbohydrate goes up and the dependence on fatty acids goes down. This means your muscles will use your glucose storage tank to fuel your workout so you can eat more carbs!

At low intensities, fatty acids are the main fuel source and only small amounts of glycogen are broken down. As the intensity of exercise increases, larger amounts of glycogen are broken down and burned for energy, making glucose the predominant fuel source.

Amino acids from protein are the lowest priority fuel, given amino acids are the infrastructure of the muscle tissue itself. In order to preserve muscle mass, the muscle will burn glucose and fatty acids before resorting to amino acids. Brilliant design.

http://ajcn.nutrition.org/content/72/2/558s.full
Triacylglycerol oxidation increases progressively during exercise; the specific rate is determined by energy requirements of working muscles, fatty acid delivery to muscle mitochondria, and the oxidation of other substrates. The catecholamine response to exercise increases lipolysis of adipose tissue triacylglycerols and, presumably, intramuscular triacylglycerols. In addition, increases in adipose tissue and muscle blood flow decrease fatty acid reesterification and facilitate the delivery of released fatty acids to skeletal muscle.

### RE: Calories into Watts

I was going to suggest that if "calorie-reduced diets don't work", it's because of denial and/or false accounting.

Steve

### RE: Calories into Watts

Yes, well...
I regretted my post directly after posting it, as it didn't really belong here. Somptinguy and ivymike, please ignore.
This is "eng-tips" and not a dieting site.

Let's stay with something measurable (with a Fluke, Agilent or whatever) and be engineers!!!

Cheers,

Benta.

### RE: Calories into Watts

moltenmetal " Muscles aren't heat engines,

Are you sure? I think they are. Perhaps belongs in the Thermodynamics forum?

je suis charlie

### RE: Calories into Watts

gruntguru: I'm quite sure they aren't. Muscles are more like fuelcells than heat engines. Not saying that muscles don't have a thermodynamic efficiency, just that Carnot doesn't apply.

### RE: Calories into Watts

great thread! Since there seems to be some interest as to power in the cycling realm this article gives some pretty interesting data to chew on.
I didnt see any mention of how the power data was collected (strain gauges in the cranks, pedals or rear hub are common but inferred power from heart rate also exists) but still pretty amazing numbers - 313W average over ~~6hrs. That definitely warrants a "wow" in my book

### RE: Calories into Watts

The pros all ride with power meters (peddle spindle, crank arm, or rear hub) and track the numbers.
They guard the data closely, but from time to time they release portions of the data.
These guys will ride 85-150 mile stage, it will take 3-6 hours, and average power output will be in the range of 300-500 W. So over the entire stage you are talking 4000-7000 kJ. I get tired thinking about it.
I can manage 150W all day, but anything over 400W in a 10 sec sprint and I die.

http://www.trainingandracingwithapowermeter.com/
http://www.hunterallenpowerblog.com/2015/05/power-...

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Calories into Watts

I have a copy here og Grier's Mecnhanic's Calculator from 1835. It has some interesting tables on pull forces and speed. One Sir John Leslie gives us an equation for for the pulling force of a horse at speed V. in miles per hour.

F = 3/4(12-V)2 lb

#### CODE

Velocity of horse    Pull
mph                  lb    Horsepower
0                   108       0.00
1                    91       0.24
2                    75       0.40
3                    61       0.49
4                    48       0.51
5                    37       0.49
6                    27       0.43
7                    19       0.35
8                    12       0.26 

My grade 10 physics teacher told us that a horsepower was 550ft.lb/sec, and he estimated that the horse used to test this died of old age some twenty minutes after the test.

Grier quotes someone named Gregory, claiming that a man at rest can exert 70lb, and can walk unloaded at 6ft/sec. This man can draw 31-1/9lb at 2ft/sec.

--
JHG

### RE: Calories into Watts

I read quite some time ago that when he quantified 1HP he simply did a test on a standard horse and multiplied the number by 2 for marketing purposes.

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