Maximum Power (in Watts) output is typically calculated as:
Power = (Max Freespeed / 2) * (Stall Torque / 2) *
Conversion Factor
Maximum Power (in HP) can then be calculated as:
HP = Power / 746
Really, horsepower is a misleading and useless number. Power output (in Watts) is by far a better and more accurate comparison. When coupled with efficiency, it's the best comparison.
Horsepower and Watts are the same thing, but HP is an Imperial Unit and the Watt is the SI (metric) unit. SI units are by far better units to use, because you will use less conversion factors (less screw-ups) when moving between Power output and Electrical input. However, remember to mention you are using SI units in all your calculations or you might end up like NASA!