We do not know, but you can request it at Share Ideas | Garmin. I did it a few years ago too, but likely the number of people requesting it, is not sufficiently important.
If you asking for energy expenditure to be quoted in kJ instead of kCalories then yes it is a bit odd. kJ is the SI unit of energy, and according to wikipedia Kilojoules (kJ) are primarily used instead of, or alongside, kilocalories (kcal) for food labelling in Australia, New Zealand, and South Africa. The European Union, United Kingdom, and China also frequently use kJ on labels as part of the International System of Units (SI).
Ideally there would be the option to show either or both units.
I don't think it is a trivial change at all as it would require Garmin to update the UNITS section in the User Settings, and then propagate the conversion to all the places on all versions of software and all devices where calories are shown. And you can be sure that Harry Hardcode has been hard at work all over the Garmin software estate, in which case it might take years for every instance where calories are currently quoted to be revised. And during that time there will be inconsistencies which will generate more support calls as users experience the unintended gradual roll-out. It may also require plots and specific graphical formats where it is used to be reworked. An example of how Garmin don't re-use common code to do the same thing even within GCM can be seen from the various different date formats that are used on the abscissa of plots such as Sleep, Weight, Race Predictor, VO2 Max, Running Tolerance etc.
However, I think kJ should be implemented and as such I have also submitted an idea based on this post.
