Tony, calibration represents a real problem, as there is no international standard definition of horsepower. The closest definition is that one horsepower equals between 735.5 and 750 watts of energy. Basically the power measured relates to the amount of work delivered over a given period of time.
Manufacturers tend to quote their outputs on the basis of what best flatters their product. As a result it is difficult to be sure of what is fact, when making comparisons between what is marketed.
From a practical point of view, you are only interested in being able to accurately record reliable, repetitive comparative figures, so that you are exactly aware of the effect of adjustments and modifications. There is no real requirement to express a figure in horsepower, so that as a basis you can work on the figures I have given, using known weights as Adam has described. Quote your figures as Tony HP.
