Calories used.

[Webbo2]

I‘ve recently done 2 identical interval sessions on the turbo. However today I used my power meter pedals and the calories used are a lot lower.
First session average heart rate 137 max 164 calories used 678. Second session ave h/r 137 max 167 calories used 388.
This using a Wahoo bolt unit. So I’m a bit puzzled why the calories would be lower when using the power meter as my heart rate is virtually identical.

 

[Ming the Merciless]

What does it say the kJ are? With power data that comes from power meter pedals and directly relates to the calorie calc. When you have power, you have a better estimate than you get from HR alone.

 

[Webbo2]

The KJ are 388 same as the calories. This is the first time I used the power meter on the turbo but I have noticed since getting it the calories used is much lower than when they were estimated by heart rate.
As I’m now not burning as many calories it’s clear as to why my trousers are feeling tight.😏

 

[presta]

Calorie consumption is approximately proportional to the square of HR, so you'd expect calories to be equal if the RMS of your HR is the same between rides, not the linear average.

 

[ivegotabike]

Power data is based on output at the pedal and is a more accurate input for the calories used calculation than HR.

I don't know if the Wahoo calculation takes into account normal metabolism, or if it is just saying that you rode X seconds at Y watts and that equals 388 calories.

You can do some sense checking to see how different variables affect things. The https://sport-calculator.com/calculators/cycling/cycling-watt-calorie-calculator has conversions watt > kcal kJ > kcal and kcal > watts

As per its method statement, it presumes 24% efficiency of food energy to work at the pedal:

"Method: work (kJ) = watts × seconds ÷ 1000. Your muscles turn about 24% of the food energy they burn into work on the pedals (gross efficiency), so kcal = kJ ÷ (4.184 × 0.24) ≈ kJ × 1.0. That is why 1 kJ on your power meter is about 1 kcal burned. Body weight doesn't matter here: the watts already measure the work."