Found a nicely concise quora here:

https://www.quora.com/What-was-the-horsepower-and-torque-of-the-Saturn-V-rocket
The "Horsepower" of the Saturn V was the shaft power of its gas generator and turbopumps.

There are no such components in a solid motor.

Thinking back to system dynamics, Power can be described in terms of Effort*Rate. This works for a majority of dynamic systems:

Effort * Rate

Translational => Force * velocity (actuators, etc..)

Rotational => Torque * Angular velocity (Pumps, car engines, etc..)

Electrical => Voltage * Current (computers, heaters, etc....)

Hydraulic => Pressure * Volumetric flow (drawing a blank on an example here, but the math works!)

(another reason metric is cool, because interactions between the systems are Waaay easier to work through)

Back to rockets, I agree with the quora answer, Power really depends the mass of the vehicle being lifted. Looking at McCord's calculation above, It looks like you'll have to multiply that Total impulse by the acceleration produced before you would have units of power. N*s x m/s

^2 = N*m/s.
The concept is uncomfortable to think about if you equate force with power/energy. 550k pounds of thrust going nowhere in a static test? 0 Watts or Horsepower. 0 Energy imparted to the system (ignoring heat, acoustic energy, and material compliance that is).

Buuut, if you really want to know the Horsepower curve of your rocket, take the thrust curve and multiply it by the boost velocity curve and you'll get a power curve.

For example, lets say that during my cert flight, Big SAM was going 90 m/s while its H120 was still putting out 120 N of thrust. That makes 14.4 hp (10.8 kW)

Or if I subtract the weight of the rocket from the thrust (lets say 21.85 N) that'll be 11.8 hp (8.8 kW).