On the Stages Indoor Cycling console, you’ll see two similar looking measurements, the kcal and the kJ, displayed during the ride and in the RESULTS summary at the end of the ride. The kcal and kJ are similar measurements that represent the energy used during your workout. Below we'll explain the difference between these units and why they are displayed.
What is a kcal?
A kcal is a kilocalorie, or as most of know it, a Calorie (note the capital C). The kcal is the same Calorie measurement commonly seen on food packaging that shows how much energy the food you’re consuming will give you.
During your ride, both the instantaneous and the average kcal/hour are displayed on the left and right side of the screen. These measurements help give you an idea of how many kcals or Calories you will burn if they maintain that pace for an hour.
What is a kJ?
A kilojoule is the metric equivalent to a kcal and also represents the energy used and is able to be directly converted over from the power readings on the bike. Since we're measuring real power in watts, we can accurately calculate the kilojoules of energy required to spin the cranks during your workout.
1 watt for 1 second = 1 joule.
To get kilojoules, we simply multiply the average power x seconds riding, then divide by 1000 to convert from joules to kilojoules. This is a nice, easy conversion from watts.
Average power x seconds riding / 1000 = kJ total
What’s the difference and why do we need to know both?
The reason we show both is that the conversion from kilojoules of energy used to spin the crank doesn’t convert identically to the amount used by the body, due to the inefficiency of the body. The efficiency level varies from 18 to 26% and since we can’t know your exact efficiency level, we use an average value of 22%.
The mathematical equation we use to make this conversion is:
kJ / 4.186 / .22 = kcal
What does this mean? It means the kJ value we mention above only accounts for the energy that actually turned the cranks, not the total energy consumed by your body. By factoring in the 22% efficiency level in the conversion to kcal, the console will give a very close estimate to the amount of kcals consumed during your workout. If you're using a USB drive to record the rides on the console, check the summary at the end of the file for a listing of all these values to keep track of the amount of work you're putting in from one workout to the next.