I hate to say this, but I agree wtih Merlin.
Think about it this way-compare Lance Armstrong with your average recreational rider (say, me)
If we both ride at 180 bpm, I am probably going 22mph on the flats. He is going like 30, and putting out a ton more watts than I am. So if we both ride for an hour, he has done a ton more mechanical work (which physics says is watts* length of time), which translates to more calories burnt. Thus, any heart rate monitor that tells you how much calories you burn can't be accurate since it doesn't take into account your heart rate. Now Polar gets around this in the OwnCal system which uses a much more complicated formula than the one above. The actually measure your heart rate variability, which they claim is a measure of your overall cardiovascular fitness and factor this into their calculation-i.e. the more fit you are, the more watts you'll put out at a given heart rate and the more calories you'll burn. This is the OwnIndex that the machine puts out.
The only way to truly know how much mechanical work you do on a ride is with a power meter.