Strava/TR calculated calories from power meter data incorrect, joules given as calories?

I wanted to know how much carbs I would need for some vo2max intervals so decided to calculate the energy needed for the interval: power in Watt x time in seconds / 1000 = energy in kJ divide kJ/4.2 to get the calories. What I noticed is that the amount of calories strava gives a ride equals the average power x time in seconds/ 1000, so not the amount in kcals at all but in kJoules, almost a factor 4.2 too high! (1 cal = 4.1868J to be exact).

Did I do my calculations wrong or is Strava really off?

Edit: looking at trainerroad workouts, TR makes the same mistake and equates Joules to calories

From a quick search the formula used is:

Energy (kcal) = Power (watts) * Time (hours) * 3.6

Thatâ€™s in joules not calories. 1 calorie = 4.2 Joules. There are 4 kcalories in 1g of carbs but almost 17kJ

Thatâ€™s assuming humans are 100% efficient. We are notâ€¦ it is actually closer to 25% so in essence cancels the other out.

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The TrainerRoad software equals your calories to the work done in kilojoules during a ride. Power (watts) x duration (hours) x 3.6 equals work done in kJâ€™s. For example, 200 watts for 2 hours is 200x2x3.6 = 1,440 kJ, which is what TR suggests in calorie burn. HOWEVER, this assumes a gross metabolic efficiency of 24-25%, which most research seems to disagree with. Most people fall somewhere around 15% lower in terms of GME, which means they expend about 15% more calories than the work produced in kJâ€™s during a workout. For me, this has proven to be much more accurate. Thus, calculating calorie expenditure goes as follows. Power x Duration x 3.6 x 1.15 = calories burned. OR, to make it easier, 4.14 x Power (W) x Duration (hrs). GME rises as the intensity does, and at FTP power we may actually reach 24-25% efficiency, but at lower intensities most people are not that efficient. The only way to know precisely would be to perform a gas exchange test, but kJâ€™s + 15% seems to fall within the accurate range according to most articles Iâ€™ve read as well as my anecdotal experience.

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As mentioned by others, this is because the human body is not 100% efficient. It turns outs that only 21-25% of the energy consumed is used to turn the pedals. If you actually were 100% efficient you wouldnâ€™t produce any extra heat while exercising and you didnâ€™t have the sweat.

I think itâ€™s just plain wrong to do it like it is, even it is in reality about right. It should say work done and probable calories burned or our best guestimate

Yes, it is â€śwrongâ€ť to assume that kcal is the energy burned by the body vs kJ is the work. The are both units of energy, but unfortunately that is how it is done in most apps. Actually, except for cycling where you have power meters I think the apps will typically only show the estimated energy burned by your body.

To make things more confusing some watches/apps will also include both active and inactive energy burn (what you consume at resting heart rate).

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Strava does display both values if you are interested.

My recent vo2max workout displayed a calorie burn of 580 cal and a total work of 591kJ

I really donâ€™t know how they could possibly get around the â€śestimateâ€ť for human efficiency - but the formula they use seems to work pretty well for me?

EDIT: Just noticed that TR does just simply use the kJ value for calâ€¦ not sure how i feel about that but I suppose its near enough? Displayed as 591KJ(CAL) in the app.

Itâ€™s not perfect, but it is as perfect as youâ€™re gonna get for a big group i guess. Or at least, good enough for a general population.

For me itâ€™s pretty accurate. Been tracking my weight, calorie intake and KJs burned for over two years now and it tracks pretty well for me. But with the margins of errors in KJ to Cal and nutritional values on food not being 100% accurate. You really have to test it for your self and perhaps adjust.

3.75 kcalories per 1g of carbohydrate but whos counting

Source: British Nutrition Foundation

Think we can all agree that it isnâ€™t this wrong though?

Like a few others have said, I think the calorie expenditure displayed is as good a number as can be expected and I definitely find it useful for fueling as it is.

So does Garmin Connect, under â€śStatsâ€ť. However, when it imports a TrainerRoad workout, it normally reduces calories a bit too.

And the power meter margin of error too. Like you, I find it (and active calories where no power meter) work for me. Iâ€™d come down in rather under estimate than over estimate.