Power Computation

It’s probably been answered before, but here goes:

On a recent ride, Zwift reported 603 Cal in 52:29. 603 Cal = 2521.7 kJ and 52:29 = 3149 second
Average watts should be 252170 J/3149 s = 800.8 Watts. Zwift reported 197 W, or less than 25% of the straightforward computation. So either the number of calories is wrong, or the number of watts is wrong, or Zwift is only counting 25% of my watts, possibly with the rest being lost as heat. So what’s up with the Energy (Cal) and Power (Watts) computation?

Yep. Efficiency of the meat engine is about 25%. That’s why you sweat buckets and can heat up your pain cave by several degrees.

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In a nutshell the calorie calculation is a one size fits all algorithm that only uses limited metrics in order to give you a figure.

Zwift is estimating your calories burnt during the exercise period, but not all of those go into the power output at the pedals.

Depending on your smart trainer, the Watts is reasonably accurate. The calories is a much more an estimate based on ‘typical’ values.

Most GPS device will also so an estimated calories expended for a ride. The estimate is much lower if I also wear a heart monitor that connects to the GPS - its data improved whatever calculation is used for the estimate. And even lower if I also have a power meter supplying actual output power data to the GPS which further improve the calories estimate. But its still a estimate. Zwift are doing much the same.

If you want the Calories to be accurate it can be done but requires you to also be attached to a lot more equipment in a lab.

calories burned is just an estimate taken from your actual power and your entered weight… it’s actually a reasonably good estimate as long as your entered weight is also accurate, but it’s not exact as there are a lot of other incalculable variables involved. 25% sounds about right for a guesstimate, even on the high side, 20% would also be a reasonable figure

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Thanks. I use a Wahoo Kicker Core (I supply cassette), so I should assume my Watts are correct and then adjust Calories.

Just a note: Calories consumed (not just those applied to the pedals) is the statistic of interest for us average (and older) Joes who are not riding to train for the TdF. I can see that energy applied to the pedals is of more interest to racing enthusiasts, who might comprise a significant number of Zwift users.

Also, the ratio I came up with of 25% is suspiciously close to forgetting to convert Cal to kJ.

I won’t bug you more on this.


Chuck LaBerge

Nice answer. Thanks.

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