To calculate power of torque, we need to know that power is the rate at which work is done by a force, and Power P= dW/dt (derivative of work W related to time t). In the case of rotational motion by a torque, the torque X produces work dW=Xd`theta` as it rotates an object by the angle d`theta` .

Therefore, power of torque  in rotational motion is expressed as : P=dW/dt= d/dt  (Xd`theta`) = X (d`theta` /dt)