If you record power and heart rate when you're cycling, then you can calculate the joules of energy delivered per heart-beat. This is a figure I haven't seen mentioned anywhere else, and I've made it possible to calculate the figure for each Strava activity that contains these data. This number will clearly vary from person to person, depending mostly on their heart capacity and muscular efficiency. For comparison, my 'typical' joules-per-beat is around 150 - 160. Note that in the calculation I subtract 60 bpm from the heart rate, to remove the effect of the typical resting heart rate.