Using a Power Meter to Calculate Energy Expenditure

 

 

2012-09-25_2053

Sean Burke ©Jinna Albright

by Sean Burke

You may have heard riders say that they know how many Calories* they burned on a ride by looking  at their power meters.   These riders simply look at the total amount of kilojoules on their computer, and then say that they burned the same amount of Calories on the ride.   While, strictly speaking, that statement isn’t true, the total work done in kilojoules is an excellent estimate of how many food Calories it took to do the ride.   Let’s examine the relationship between watts, kilojoules, and then Calories, as well as the math behind calculating energy expenditure from power output.

Steve is a pretty good cat 3 racer. He weighs  75 kg ( 165 lbs) and his threshold power (sustainable steady-state effort) is 300 watts.  If Steve  does the state time trial championships or a 1 hour criterium at his threshold power,  he’ll burn about 1000 Calories per hour.  If he does an endurance pace ride, he will burn about 700 Calories an hour.

Let’s do the math.

First, we need to convert Steve’s 300W race pace into total work.    A watt is a measure of work over time.  Specifically, 1 watt = 1 joule/second.  Since we want to get the total amount of work, we need to multiply the average watts  X 3,600 seconds (1 hour).

300 watts X 3600  seconds =  108,000 joules, or  1,080 kilojoules.  So the total amount of work Steve did in the hour was 1080 kilojoules (KJ).   Let’s go ahead and convert that to Calories.  Calories and joules are both units of energy, and there are exactly 4.184KJ in 1 Calorie.  1080/4.184 = 258, so Steve did 258 Calories worth of work, but he actually burned way more calories than that.

You see, the human body is only about 24% efficient at turning food energy into mechanical energy, and the rest is essentially lost as heat.    While that percentage isn’t exact, and is dependent on several factors,  it is going to be VERY close to that 24%.      The amount of KJ in a Calorie is a given, and while the 24% metabolic efficiency isn’t exact, I would argue that the  +-2-3% accuracy of  your power meter contributes more to any error in the calculations than does this assumption of 24% efficiency.   So we can simply multiply our mechanical work calories by 0.24 to  get our required food Calories.

258 Calories  X 0.24 = 1,075 Calories.

Steve burned 1,075 food Calories by going at his maximum steady state for 1 hour.  You may notice that 1,075 Calories is very close to the 1,080 KJ  of work that Steve did.     That is why you’ll often hear people say that the number of kilojoules done during a ride is the same as the number of Calories burned.

So we know how many Calories a cat 3 racer will burn during a 1 hour effort at race pace.   What about an endurance pace ride?  An endurance pace ride could be a short easy ride, or it could be a 4-hour ride with some hard efforts.   Either way, the average power is going to be about 2/3 of threshold power.   In fact, a 4-hour ride, no matter how hard it is, MUST fall into the endurance range for average power. Even if there are several efforts over threshold power,  the recovery required in between efforts will bring the average down into the endurance range.   Andy Coggan defines the “Endurance Zone” at 56-75%.  For demonstration purposes, we’ll use 66%, right in the middle of the endurance zone.    For Steve, our 165lb rider with a threshold power of 300 watts, we would expect him to average about 200 watts, and we can use the same mathematical formulas as before: 200 X 3,600=  720,000 joules, or 720 kilojoules.  We don’t really need to do the rest of the math to know that Steve  is going to be burning ~720 Calories/hour during his endurance paced ride.    If you have a higher threshold power, you would burn more at race pace, and more at endurance pace.  If you have a lower threshold power, then figures would, of course, be lower.

If you have a power meter,  the whole thing  is pretty simple.   Just look at the total number of KJ that you’ve done, and it is an excellent estimate of how many calories you’ve burned.

Do you have any questions or topics you would like me to cover?  Comment below.

 

Sean Burke is the head coach for Crank Cycling in San Diego, CA.

*I use Calories with a capital “C” to denote kilocalories.  Food Calories are almost always expressed this way.  If you read the nutrition label on the back of a food product, and it says a serving has 100 calories, they are actually talking about 100 kilocalories.   1 calorie = 4.184 joules, and 1 kilocalorie = 4.184 kilojoules.

 

 

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Comments

  1. Nick says

    Hey man, awesome article and very insightful.
    But I think you made a typo when calculating the Calories needed to race a crit, you said 258 Calories X 0.24 = 1,075 Calories. But it should be divided
    (258 Calories / 0.24 = 1,075 Calories)

  2. shaun wallace says

    Nice article Sean.
    My question: Given that we burn calories even laying on the couch picking our nose, what is the DIFFERENCE in calories burned per hour if we go for a ride instead? Or how about if the missus persuaded us to go walk around the supermarket with her, what additional calories could I explain to her I’d burn if I went for a ride instead?

    • says

      Great question.
      Those calculations are only for the work that you are putting in to the bike, and don’t include your BMR, or what you would use just sitting with your feet up watching the cat whisperer. So these numbers ARE the difference.

  3. Matt Maxfeldt says

    Thanks Sean, really useful info. I’m wondering how these calories burned relate to fat loss. Are threshold-pace calories burned the same as endurance-pace calories burned when it comes to dropping weight? If not, can you explain? Thanks!

  4. says

    Matt,

    That is a short question with a long answer. But the short answer is, greater energy deficit = greater weight loss. So get out there and do more KJ worth of work if you want to drop some lbs. The longer explanation may be a good topic for a future write up.

  5. David says

    Great article, I was curious is there a way of combining the calorie estimate from the power meter with the calorie estimate from a hear rate to get an even better approximation?

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