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.
*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.