“Almost all current power meters report the amount of work you have performed in joules in addition to measuring and recording your power in watts. Joules (J) and kilojoules (kJ) are therefore a measure of energy expenditure, or work performed. Here in the United States, however, this is usually measured in kilocalories, or Calories (1 kilocalorie, or large Calorie [with a capital “C”], is equal to 1,000 small calories [lowercase]).
By definition, there are 4.184 kJ per Calorie, so at first glance it would seem that to determine your energy expenditure using power-meter data, you would simply divide your total work in kJ by 4.184. However, this is not correct because power meters measure external work production, not the amount of energy needed to perform that work. Most of the energy expended during cycling is actually converted into “waste” heat that must be dissipated to the environment, with only a portion available to actually turn the pedals. The relationship between work performed and energy expended depends upon your thermodynamic efficiency (i.e., your ability to process food and convert it into energy) when cycling, which, for most trained cyclists, is on the order of 20–25 percent.
Thus, to estimate your energy expenditure (in Calories, or kilocalories) from the amount of work performed, using a power meter, you would need to first divide your total work in kilojoules by 4.184, but then multiply this result by either 4 (if efficiency is at 25 percent) or 5 (if efficiency is at 20 percent). These conversion factors tend to simply cancel one another out, such that you can also take the value for the total work performed in kJ as an estimate of your energy expenditure in kilocalories (or Calories). Although the exact relationship between kJ and kcal is not one to one, it probably is not worth worrying about any error this assumption creates, since an individual’s efficiency can only be readily determined in a laboratory setting, and can vary depending upon the intensity and duration of training, environmental conditions, and other factors.”
Excerpt From: Hunter Allen & Andy Coggan PhD. “Training and Racing with a Power Meter, 2nd Ed.” Training and Racing with a Power Meter, 2nd Ed.”
As one of my New Year’s Resolutions, I have resolved to re-read some of the best and most respected books on the subject of fitness, training, and exercise physiology.
I’m starting this re-education with a book that many in the cycling world consider the “Bible” of training with a power meter, Training and Racing with a Power Meter, 2nd Ed. – Hunter Allen & Andy Coggan PhD.
As I read I’ll share excerpts that I believe are pertinent in the world of indoor cycling and the ever growing use of power meters and group display.