Should I be using machine calorie counts or Fitbit calorie counts

[YachatsMike]

Calories confuse me.  I use three exercise machines that report calories consumed.  My rowing machine derives calories from intensity (Watts) of the row, my recumbent bike from pedal speed and resistance level (Watts), my treadmill from pace and slope (Using an ACSM formula).  My Fitbit, though, derives Calories using some heart rate-driven algorithm.  What that means is that in the first 10 to 15 minutes (first three 5-minute Laps or so) of an exercise, while my heart rate is still rising into the aerobic zones, the calorie readings are significantly lower on the Fitbit than on the machine computers.  

I think I should believe the machines (Watts are watts) but I would be interested in hearing an argument for the Fitbit  values.  I'd also like to hear other people's methods for turning those values into Mets, the measure used by HHS for the Physical Activity Guide  (I'm  using Cal/minx200/Wt(Kg)/3.5)

[Triletics]

@YachatsMikepower provides probably the best estimate. As you noticed, when your HR is still rising, you're already outputting the same amount of energy. This is the reason why calorie estimate based on HR works best for steady-state cardio with longer duration. Then initial rise and cooldown drop are not so significant while steady HR throughout the exercise can provide quite a good estimate. This doesn't work that well for workouts involving intervals or weight training as HR tends to rise and drop all the time and the heart, unlike the immediate power, isn't as responsive to changes of effort. Whichever estimate you use (I would go with power) it's up to you. I also use running power. Many watches like Garmin, Apple or Polar and foot pods (Stryd) support it nowadays, although while the model is physically correct it's only the model (a real-world running power isn't measurable) which requires some adjustments (usually, just scaling up or down).

[Triletics]

@YachatsMike I did a quick check with a Concept2 rowing machine and my Garmin watch uses power from Concept2 to estimate calories. It does the same for cycling when power meter is available. In other words, power seems to be taken over heart rate when considering energy expenditure.