I have a ProActive Cumulus HRM - (don't ask why. I am not somekind of weird training freak, I just find it all interesting)...anyway this particular model has a 'calories burned' feature. And given that it calculates this from pretty basic info (max HR, resting HR, time exercising, age, weight, sex, average HR) I was wondering how accurate would it be? Or will it be so far out that I may as well ignore it?

Tom - just interested.

And now the science bit

http://www.halcyon.com/gasman/energy.htm

Wouldn't mind knowing, I've got the equivalent on a Polar M52HRM

It's going to be an estimate... like any other system. You can't measure what's going on in the body the way you can with a machine. How good is the model they're using? Probably pretty crude. I find it interesting (I have a Polar M51) but if I were to start training seriously I would use calories, time and distance to measure effort.