Following on the heels of the post “What’s a Watt?” I wanted to jump off that and get a little more info out there on one of the things you can do with a power meter. A lot of people, (especially in the off season) focus on losing weight and getting a better body composition. To accurately count calories, you need an accurate count of calories burned. Any GPS will give you a number of calories burned for a given workout, but sometimes that is a bit of an arbitrary number. This is where a power meter comes into play!
We already discussed what Power Meter’s measure (watts= torque + rotational speed). If you haven’t read my post on this, this is a good time to go back and read that. Don’t worry, I’ll wait.
Obviously, measuring watts gives you a really good measure of how hard you are working, and from that your cycling computer can measure the total amount of work you did over the course of your ride. This measurement is called a “KiloJoule” (kJ). A kJ is simply a measurement of energy. It is actually used in measuring electricity too.
The actual definition of a Joule (kJ is 1000 Joules) is the amount of energy required to move a newton of force for one meter. Now, that obviously doesn’t mean much to us every day because we are not in the habit of measuring newtons of force. Thankfully, our cycling computers do this for us. So, we can know the amount of work we have done by looking at the kJ’s.
It is actually really easy to translate that number into calories. First of all, it is technically a KiloCalorie, but in America we just shorten that to calorie… which is not technically correct, but it’s so common we just run with it. Officially, 1 calorie is burned to produce 4.2 kJ’s of work. What makes this super simple is that no one is 100% efficient in burning calories. That means not all 4.2kJ’s of the work potential is actually transferred to pedals. Roughly 3/4th of the work is lost in heat from the burning of the calories. In that case it comes down to almost a perfect 1-to-1 calorie to kJ ratio.
As an example, my ride last weekend was 161w (Normalized Power) for almost 1:30. That came out to 741kJ’s of work and 737 calories burned. Now, I had software to tell me all of that (TrainingPeaks and Garmin Connect) but if you were just going from a simple conversion, 1-to-1 is close enough.
You can see from this that a power meter can be really useful for getting a good idea of how many calories you are burning in your workout. This is important both for someone trying to lose weight as well as when making a race nutrition plan.
How do you use the data fields on your power meter? What other data points would you like to hear explained? Let me know in the comments or on twitter!