As a workaround, I added the conversion to kJoules int the local overrides of Garmin Connect:
I have a Fenix2 and am bewildered why Kj isn't an option. What a waste of time trying to find that. It reminds me of a baby-infrared thermometer I bought - it simply wouldn't read out in °C - clearly the s/w developers haven't travelled much and aren't international people. How unimaginative!!