Everyone thinks about power when talking about motors. For decades we have talked horsepower, with our shaft-turner AC motors, as well as the engines in our cars. For servo motor sizing, power is the very last consideration.
There are fundamentally 3 things that determine the right servo motor for an application: speed, torque, and reflected inertia. Most people think about torque and speed because we get the nice curves generated by servo motor manufacturers. Yaskawa servos guarantee torque at the shaft, which is shown in speed-torque curves.