My understanding is that while cycling 1kJ of work equates to roughly 1Cal burned. I just completed an FTP test and Zwift recorded 267kJ of work, which seems about right, but it said I burned 752 Cal. Why would it be so high? I did not use a HRM.
Actually 1 Cal = 4.2 kJ. So your 267kj should really be 63 Calories (which does seem low for a FTP test workout)
I must admit it is all a bit confusing to someone who has only ever used calories like myself
Correct, but gross metabolic efficiency for cycling is around 25%, which gets you back to 1 kJ = approx 1 Cal. Here is a good article that goes into that in more detail. https://blog.trainerroad.com/calories-and-power
Your “work” is power really which is measured in watts, I think anyway
Almost. I don’t say that to correct you, I just wanted to clear up my own understanding. A watt is a unit of power. It is an element of work. Work happens when you expend power over time and is measured in Joules. Or in reverse, power is the rate of work being performed (J/sec.). Producing 1 watt for 1 second equates to one J of work. So, riding at an average of 200 watts during a 20 minute FTP test would expend 200 watts x 20 min x 60 sec/min or 240 kJ of work.