# Solved: Ratio of Kcal (energy used) to KJ (work done)?

I found something in my logs I don’t understand. On the podcast, @Chad has said that Kcal of energy use and KJ of work performed are roughly equivalent since 1 Kcal = 4.18 KJ, but the human body is roughly 24% efficient so needs to burn 4 Kcal to actually do ~4 KJ of work. That matches everything else I’ve ever heard and makes sense.

But when I look at the 51 outside rides in my logs (data from Quarq PM, recorded by Wahoo ELEMNT, uploaded to Strava, analyzed on Veloviewer), I see very different numbers. The average ratio of Kcal to KJ is shown as 2.20 with a standard deviation of 0.26, so ~95% are between 1.7-2.7 Kcal/KJ. My big rides are around 1500 KJ of work, and there’s a big difference between “refueling” 1500 Kcal or 3000+ KCal!

Are Strava (or my ELEMNT) just guessing, and not very well at that? Or are those numbers a reasonable representation of the additional “energy cost” of riding outside in the Florida heat/humidity/sun and having to cool the body down much more than usual?

various people have reported this problem with Wahoo Elemnt and Bolt. It may have been fixed with a recent firmware update? I don’t have a Wahoo head unit, maybe someone that does will stop by and help.

Strava will show you actual kJ in terms of work which is the number approx equal to the calories burned. Wahoo used to do calorie estimation based on a combination of HR, body size, etc, even if you had a power meter. A recent update has since enabled the products (or at least the ELEMNT BOLT, and I assume ELEMNT) to utilize power data for calculating kJs.

Correct - until a couple of months ago Wahoo would report calorie burn based on their HR algorithm, even with a paired PM. They corrected it in their firmware a few releases ago and it now correctly matches.

This was purely a Wahoo problem and has been fixed. If you haven’t updated your firmware recently do so and it should be resolved. If you have updated recently you should see a match on your more recent rides

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Are there “extra” calories that are burned from being on the bike, though? For example, if I’m descending on a mountain bike, I’m unlikely to be putting any work into the pedals, but I have to be burning some extra calories, right? Or on a road bike, my HR doesn’t quite get back to baseline, even coasting down a gentle slope.

In terms of calorie balance for weight loss, etc. is it only about the kJ that go into the pedals? If so, why?

Hi @maletero, as everything else, the calories burned are an estimated value based on lab studies and extrapolated to the real world (ramp test is one of does examples).

The answer to your question is it depends on the device. You are asking if the device accounts the rest metabolic rate.

ALL the numbers given by your device, independently of the metric given, always have an accuracy related to them and that’s important to keep in mind.

Edit: Just to make it more clear. Is not your heart rate that determines the calories being burned. It’s assumed that at a certain heart rate, depending on genre, age and body weight, you are burning x amount. The PM eliminates part of the guessing.

Thanks for that. As I understand it, BMR basically the energy used to keep you alive. I think on last week’s podcast they talked about it as something like the energy used if you’re awake but doing literally nothing. That site you linked seems to support this (is that the link from the shownotes?).

It helps, but I still have a question about whether on a ride we’re dealing with:

Ride calories = BMR + kJ

or

Ride calories = BMR + kJ + other cals

Other calories might be stabilizing muscles, increased HR for whatever reason, etc. But maybe the energy that goes into the pedals is so much more than the “other” cals that the kCal = kJ rule of thumb is enough.

(My head unit is a Wahoo Bolt and don’t have a power meter, yet.)

IMO the “other” calories are negligible. (Edit: to address @DaveWh comment below. Assuming that you pedal most of the time. No so true in MTB as an example.)

Again, different apps will use different ASSUMPTIONS and will have different results. Think about yoga, how can anyone put a number on that?

Because you don’t have a PM, your Wahoo is most likely estimating based on your heart rate, gender, age and body mass. I don’t know the ins and outs of wahoo but they might even use your speed and gradient to help the guesstimate.

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Depends on what you’re riding. On MTB, it’s very meaningful.

At the end of the day, the best way to measure progress towards a target weight is the scale. As calculations of kJ, kCal, and calories consumed are prone to pretty sizable error.

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As others have said Wahoo used to do it purely on Heart Rate, even with a power meter. That was updated in the last few months. You could always select to display Kj, and it always pushed through Work done to strava/ connect when I uploaded.

For rides without power, I use my Garmin watch for calories, as my n=1 is using that for running and swimming and non-power bikes over the years has given me the expected weight loss results for my food intake (that’s across 310, 920 and now 245). The Wahoo calculation has always been way to high for me. It took a few weeks of non-expected weight loss to cop it really though.

When I mountain biked previously, and now with my Gravel Bike without power, I remain to be convinced there’s not extra burn from downhills/ general body impact. GRX crankset so I can move over my 4iii to my Topstone is on the list though!

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I edited my post above as I was mainly thinking in a general road setup with a fairly constant output. Trying not to write an essay as well…

To be clear, you bat an eyelid and there is and there is energy consumption associated with it. So yes mtb downhill will make you spend more cal than laying down on your bed.

My point is more on how your device is able to capture that.

Accuracy and precision are not the same thing. And is important to understand what your device is giving you.

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Yep, my discrepancy appears to have gone away in early June. Thanks to everyone!