As you’ve mentioned, the servo motor calculates by Watt. It is because the servo motor’s torque is constant regardless of speed.
As for the stepping motor, it is difficult to calculate by Watt because the motor’s torque varies depending on speed.

However, the capacity at each speed is needed to reveal the number of Watts. The following table is about interchangeable capacity from the servo motor to the step motor at around 1000rpm. In the case of the rpm exceeding 1000, the torque might lower below the capacity, so please be cautious of this.