It's not easy, you need a base line calories per hour, weight, height, age and gender to make any energy measurement turn into a fake kilo calorie (kCal) measurement.
The classic example of how NOT to do it, is the Concept2 rowing ergometer (they assume everyone is a 25% efficient energy burner, burns 300kCal/hr doing no active work and weighs in at 80Kg - you'll see a immediate flaw there).
http://eodg.atm.ox.ac.uk/user/dudhia/ro ... #section11
It's in no way as simple as F=ma.