Calories calculation

[bubak]

I am using PP with Garmin Edge 830 and both devices always report the same amount of burned calories. Is this just the coincidence or one device feeds another?
Another problem is that these values are only slightly above half of the values calculated from HR by sports watch, Polar Beat or Edge without PP. The new values seem more like work done in kJ not burned calories.
So if PP calculates burned calories could somebody explain this big difference?

[Velocomp]

1) Calories are derived from power integrated over time (this is the physics definition of energy). Garmin uses PP signal to calculate calories. The number it reports should be very close to PP

2) HR devices are notoriously bad at calorie measurement, because HR straps don't measure power. They guess at calories, and the guesses are very poor.