General Discussion Triathlon Talk » Is this an accurate conversion of Watts to calories and viceversa? Rss Feed  
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2013-11-07 11:16 PM

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Subject: Is this an accurate conversion of Watts to calories and viceversa?
Let's say I want to burn 565 calories in one hour. That's 565000 kilocalories into seconds (3600 in an hour) giving a power of 157. So if I have a Wahoo Kickr and I set it to 157 in Erg mode and pedal for an hour I can assume I burned 565 calories? I read it was weight-independent. A big muscular guy is of course going to have an easier time pedaling that particular power than a less muscular smaller female.

You can also grab power, i.e. 200, and multiply by # of seconds, 3600 in an hour, and that's calories burned, or 720,000, or 720 Calories. Also weight independent.

Since the Kickr doesn't have the downside of different tire pressures and clamp positions, it should be spot on each and every single time right?


2013-11-07 11:55 PM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Hmm...I don't think you can make a generalized statement like that. The amount of energy to move object that weight X a distance of Y should be fairly well known (ignoring coefficient of friction and such). Problem is that we are not 100% efficient. If I remember correctly, we can use about 1/3 of the stored energy we have and convert it to work while the rest is wasted as heat. I'm sure someone else will chime in with better details. Also, a less fit person is probably going to have a harder time doing the same amount of work as a fit person and thus will probably be burning more calories.
2013-11-08 2:01 AM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

A Watt is one joule per second.   One kcal (your food calories) is 4200 Joules.  

 

If you burn 200 Watts (J/s) x 3600s you get 720,000 J  or, 171 calories. (if you're 100% efficient.   You're not)

 

If the 30% figure is correct, then you burn about 500 calories, or so.

 

 

2013-11-08 4:13 AM
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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Why would you want to structure a workout to burn a certain number of calories? You should instead be structuring workouts to elicit a specific training response and, if you are counting calories, then do so after the workout.

In terms of determining caloric expenditure from power, what you want to do is:

Power * time = work in joules
Work in joules / 1000 = work in kilojoules
Work in kilojoules / 4 ~ work in kilocalories
Work in kilocalories * 4 ~ metabolic cost in kilocalories

The conversion is fairly straight forward since 4kJ is approximately 1kcal (or 1Cal) and the body is about 25% efficient when cycling. So basically you can assume that the work in kJ is the same as the metabolic cost in Cal.

Shane

Edited by gsmacleod 2013-11-08 4:15 AM
2013-11-08 12:47 PM
in reply to: #4894865

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
I'm not training, using it strictly to burn off calories bto lose weight.
2013-11-08 12:51 PM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Originally posted by GatorDeb

I'm not training, using it strictly to burn off calories bto lose weight.


Better to use a HRM to calculate calories burned-- it won't be completely accurate but there are too many errors in using power to convert to calories.


2013-11-08 1:04 PM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

You say your goal is weight loss.  The truth is, weight loss much more about what goes on in your body AFTER your workout than what goes on during.  The goal is to create a better metabolism through diet and exercise in general.  The actual calories burned during just that time you're working out is only a small part of the equation.  The point is, don't waste too much time worrying about things like trying to calculate exact number of calories burned per session.  

Just know that you are burning calories when you train and focus more on the quality of the workouts and diet if weight loss is your goal.

2013-11-08 1:14 PM
in reply to: noelle1230

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

Originally posted by noelle1230

Just know that you are burning calories when you train and focus more on the quality of the workouts and diet if weight loss is your goal.

This.  You're trying to focus on a leaf.  Look at the forest.

2013-11-08 2:06 PM
in reply to: JohnnyKay

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
gs - thanks, that makes it much more simple to calculate.

Power * 3.6 = calories per hour.
Calories per hour / 3.6 = power.

2013-11-08 3:16 PM
in reply to: jennifer_runs

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

Originally posted by jennifer_runs
Originally posted by GatorDeb I'm not training, using it strictly to burn off calories bto lose weight.
Better to use a HRM to calculate calories burned-- it won't be completely accurate but there are too many errors in using power to convert to calories.

I may be completely out to lunch but I thought the opposite was true.

2013-11-08 3:21 PM
in reply to: axteraa

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

Originally posted by axteraa

Originally posted by jennifer_runs
Originally posted by GatorDeb I'm not training, using it strictly to burn off calories bto lose weight.
Better to use a HRM to calculate calories burned-- it won't be completely accurate but there are too many errors in using power to convert to calories.

I may be completely out to lunch but I thought the opposite was true.

Power is more accurate generally. However, neither method is that accurate because everyone has different rates of efficiency, outdoor temperature is variable, other stress factors are variable. Simply put, there isn't a great way to accurately measure this. You would be better served by just eating your normal diet, and observing your weight over time. If you are gaining weight, eat a little less. You'll zero it in eventually, and you'll learn about how your body functions.

Sorry that this isn't a simple math calculation to lose weight, but the reality is that weight is more controlled by diet than number of calories burned. If you want to lose weight you need to eat better, and maybe eat less.

Side note, it is often difficult to lose weight in period of heavy training. It is harder to recover on a calorie deficit. I tend to try to lose weight well before my race specific training starts for that reason. I generally will even gain a bit when hitting my peak weeks.

Good luck!



2013-11-08 3:58 PM
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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Good answer, Dave- and I realize my reply about HR vs watts was a bit too simplistic.

I find it easier to normalize calories burned vs. heart rate because I have more experience with it, and power always seems to be a bit harder to normalize. That said, I wouldn't go into a workout with the intention of burning x-number of calories-- that seems backwards to me, too. I also agree completely with Dave's comments about diet.

Deb- if you are interested in losing weight I don't think you should put too much faith in ANY exercise-based calorie-calculations, since these will always be skewed by your fitness and other personal metrics. A variance of even 100 calories per day would be within the error of calorie calculations (of exercies and food) yet will make a big difference in your weight loss. So track your personal data and then figure out where you need to cut back on calorie intake.

Edited by jennifer_runs 2013-11-08 4:03 PM
2013-11-08 5:58 PM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

Originally posted by GatorDeb I'm not training, using it strictly to burn off calories bto lose weight.

 

Ahh.... then "titrate" your food until your beltline is the right size.  :^

And keep working out. 

2013-11-08 6:41 PM
in reply to: moondawg14

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
I've lost over 100 lbs, so I know a bit about weight loss Just wanted a quick n dirty way to figure out what power to put the Kickr on when on ERG mode when I wanted to pedal for an hour while watching a movie at home.
2013-11-08 7:15 PM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Originally posted by GatorDeb

I've lost over 100 lbs, so I know a bit about weight loss Just wanted a quick n dirty way to figure out what power to put the Kickr on when on ERG mode when I wanted to pedal for an hour while watching a movie at home.


I know.

Which is why you should probably know by now that calorie calculations are largely inaccurate.
2013-11-08 7:18 PM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?

Originally posted by GatorDeb I've lost over 100 lbs, so I know a bit about weight loss Just wanted a quick n dirty way to figure out what power to put the Kickr on when on ERG mode when I wanted to pedal for an hour while watching a movie at home.

How about whatever power you are able to sustain for that hour (without destroying yourself)?  That would maximize your calorie expenditure.



2013-11-09 1:23 AM
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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Huh, didn't think of that

Ok another reason I wanted to figure it out: I'm a numbers geek and I just love formulas :D

Edited by GatorDeb 2013-11-09 1:25 AM
2013-11-09 6:11 AM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Originally posted by GatorDeb

gs - thanks, that makes it much more simple to calculate.

Power * 3.6 = calories per hour.
Calories per hour / 3.6 = power.




Wrong.

There are two units of energy, calories and joules, joules is the international standard unit, but when it comes to diet, people think in Calories. And to make things more confusing there are two "kinds" of calories, Calories and calories! Yes, the upper case Calories is 1000 calories, that is abbreviated: 1Cal = 1kcal. The k is for kilo, greek for 1000. When speaking you can't hear the upper case C, mostly we speak in Calories that is kcal, but nobody bothers to say "kilo".

Now, Watts is international standard unit for power expressed as energy per time, the international standard unit for energy and time are Joules and seconds. 1 Watt = 1 Joule/second. But 1 Joules != 1 cal. 1 Joule is 0.239 calorie. If you're reading 200 Watts, that means you're burning 47.8 calories (lower case!) per second.

There are 3600 seconds per hour, so 1 W for one hour is 860 cal (lower case) or 0.86 kcal or Cal. Your 200 W for one hour becomes 172 Cal.

But is this really a useful number? The thing is the power meter in your bike measures the power output, that is how much energy is actually converted into motion. This can be accurately measured. Problem is your body consumes more. So you can't use this number to figure out how much energy you spend during a workout. The HRM will give a better guesstimate.

Second:

About 2 months ago you completed your first IM. You have told how you had lost lots of weight and was down to IIRC 120lb. Then you stopped and now you need to loose weight again? If that is so, clearly you don't have your diet under control. You need to learn about proper nutrition and healthy diet much more than how to convert watts to calories.

And diet is much more than just calories, it's more than just fat and protein and carbs, it's about nutrients, vitamins, minerals, amino acids, simple sugars, complex cabohydrates, soluble and non soluble-fibres, and how all these things interact in digestion and metabolism.

Go see a nutritionist.
2013-11-09 6:22 AM
in reply to: dmiller5

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Originally posted by dmiller5

Originally posted by axteraa

Originally posted by jennifer_runs
Originally posted by GatorDeb I'm not training, using it strictly to burn off calories bto lose weight.
Better to use a HRM to calculate calories burned-- it won't be completely accurate but there are too many errors in using power to convert to calories.

I may be completely out to lunch but I thought the opposite was true.

Power is more accurate generally.




Power is measured accurately but it's power output, that is how much energy is converted into motion on your bike. But your energy expenditure is much larger, the body is not 100% efficient.

You can use power as an accurate measure of effort, but your HR will give better guesstimates of your energy expenditure.
2013-11-10 12:01 AM
in reply to: erik.norgaard

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
I think folks are being really hard on Deb here. She asked a specific question for a specific reason. She didn't ask to be judged for wanting to ride the bike to lose weight.

We can defer to some authority here...here is a great article on using your power meter to plan a weight loss training block:

http://www.cycleops.com/en/training/training-resources/62-a-powerfu...

In general you can use your kJ burned on your power head unit as an estimate for how many calories you burned. It's not 100% precise but it's pretty close and will be consistent for you. When you see 200W on the power unitl, that already takes into account the ~25% efficiency on the bike...in other words about 600W are being used by your body and wasted as heat and 200W are going to help spin the wheels. So the kJ seen on the power unit already takes into account the decreased efficiency.

Sure, diet needs to be taken into account as well, I'm not denying that...but why are we judging her and telling her what she "needs" to do?
2013-11-10 7:44 AM
in reply to: AdventureBear

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Thanks! That's usually how it goes, I ask a question, get advice related to something else, then people get upset when I don't follow the advice that wasn't asked for. I was going to reply but I don't think I should justify myself for something that wasn't even asked in the first post.

I haven't set up the Kickr yet, so the iPod Touch app will show both a Kj field and a Watts field, or do I have to convert Watts to KJ myself? So I really just put it as hard or easy as I want and then just copy over the Kj field.


2013-11-10 11:44 AM
in reply to: GatorDeb

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Originally posted by GatorDeb

So I really just put it as hard or easy as I want and then just copy over the Kj field.


Yes that one.

Then over time if you are tracking nutrition and your Kj burned on the bike you'll get a better feel for if it's high or low for you. IE if you are doing 350 kJ of pedaling each night and so account for 350 C deficit in your diet but are losing weight more slowly than expected, take a fraction of that 350kg. We could probably perform some sort of gradient analysis (I'm studying machine learning algorithms at the moment), but I think just using a fudge factor like 90%, then 80% etc would work. losses for tire pressre etc if you were using other trainters should be a non issue anyway, all that would change is wheel speed, but not effort. The bigger factor is where in the drive chain the power meter is . But just track everythign and make small adjustments.

FWIW I would personally find it more interesting to do SOME type of varied workout on the trainer for that hour watching TV, but of course...if the other option is not training and just sitting on the couch...spin away. Also, it's a lot easier to follow a plot line if you are not drooling from exertion.
2013-11-10 12:33 PM
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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
Originally posted by erik.norgaard



Now, Watts is international standard unit for power expressed as energy per time, the international standard unit for energy and time are Joules and seconds. 1 Watt = 1 Joule/second. But 1 Joules != 1 cal. 1 Joule is 0.239 calorie. If you're reading 200 Watts, that means you're burning 47.8 calories (lower case!) per second.




That's correct, but my understanding is that the kJ displayed on the head unit is the actual WORK done, or work that reaches the pedals and makes the bike more forward...ie work as defined by physics. However the body is also burning off ~75% of that work as heat or inefficiency. (ie that's the leftover from the 0.239 calories... .761 calories burned off as heat. more or less )

So when you read the head unit of work performed, 1KJ ~ 1kC. It has already eliminated the .761 calories burned because they are not producing forward movement, just heat.

Interesting, that's also the reason pros can ride in extremely cold temperatures, like TDF mountain passes requiring so little clothign...their watts are so high that the wasted watts as heat produced is much higher than what us mortals can do. if I'm averaging say 150W climbing a hill, I"m producing 450 W in heat. If a pro is climbing the same hill at 400W, he / she is producing 1200 Watts in heat!

Anyway, just read the heat unit and you're as close as you can get. Track the info and learn how much over or under your typical expenditure is...it will be pretty close.

Edited by AdventureBear 2013-11-10 12:37 PM
2013-11-11 12:18 AM
in reply to: #4894865

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Subject: RE: Is this an accurate conversion of Watts to calories and viceversa?
I haven't decide which service I want to go with. It's not that I want to watch TV while I work out, I want to work out while I watch TV I watch Biggest Loser and Grey's Anatomy weekly and I'm too fidgety to just sit there. Same thing if I decide to watch a movie.
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