When we talk about energy and exercise, we often hear people talk about Calories. While the term Calories is used, the actual unit of measure from food energy is the kilocalorie (kcal). The SYSTM app uses calorie calculations based on power produced rather than on heart rate because we believe this is more accurate. If you don't have a power source connected, the app will use your heart rate and body weight to calculate calories burned.
What is a kJ?
The kilojoule (kJ) is an energy unit, like the kilocalorie (kcal). The kJ is calculated based on power expenditure for a given duration. Joules= Watts x time (seconds). There is that conversion factor, where 1 Kcal = 4.184 kJ. However, that conversion factor doesn't tell the whole story.
Why are kJ and kCal used interchangeably at times?
We can say that 1 kcal = 1kJ when talking about riding bikes due to the efficiency of humans. When converting food energy into movement (like pedalling), humans are only around 24% efficient. Every 1kJ of work completed externally (pedalling) requires around 4kcal of energy (or about 16.7 kJ of energy). The rest of that energy is released as heat, which is why exercise makes you sweat; your core body temperature is increasing. To put the heat production in another context, riding at 250W creates roughly 750W of heat, which is the same as most toasters.
Now not everyone is 24% efficient. The actual conversion rates we have measured in our lab range from 0.9 kJ = 1 Kcal up to 1.12kJ = 1 kcal.
So using this 1:1 formula will still under and overestimate calories for people, but it is significantly more accurate and reliable than heartrate-based calculations.