Calories from power meter Quarq

Just pay attention to power/kilojoules. All the rest do not matter.

Exactly. Did an easy 2hr 45 min ride last Friday without HRM and power meter and Garmin Connect estimated 2344 kcal. The ride was uploaded to Strava and they estimated 1200 kcal. When I ride with my power meter with a much higher intensity than last Friday I'm around 600-700 kcal/hr. I only trust the calories calculated from my power meter data all the rest is nonsense.

Good to know I am not the only one! But the craziest thing is that if I compare total work in Kj, there is no such a difference. I get 950kj with power meter and 1040kj estimated using a HRM. Then, why that difference in calories?

Calories and power produced as measure by the power meter are two different things. Much like a car engine where 75% of the energy contained in the gaz is wasted and only 25% get your car moving, the human machine does not perform much better in that regard.

The power meter measure the power produced mechanically to propel the bike. A power meter is very precise at this. The strava estimate not as much as it does it based on speed, elevation profile and rider weight but it cannot take into account the wind and other things.

Calories are a measure of the energy spent by your body. A large proportion of those do NOT propel the bike. In fact only about 25% of the energy you expense propel the bike. The rest goes to your metabolism, produce heat, non-productive movements (such as lifting your leg every pedal turn), etc. Even fitness plays a role i.e. fit people can produce more work with the same energy expenditure. All of those are not measure accurately and only estimated.

So despite the power meter help with the estimation of calories you would be mislead to believe it actually measure it. It does not. Calories remain estimated and yes this is not all that accurate. But it does not need to be really. While one can train much better with an accurate power measure on-the-bike, you don't train to calories.

This explanation of calories vs kj has nothing to do with a heart rate monitor's estimation of calories. The explanation for that is fantastical terrible formula guesswork that should be disregarded as silliness. That is it.