Use a Ideal Torque Sensor, connect it in series with the compressor R port, and now you are measuring the shaft torque. Multiply the torque by the angular velocity, you'll get the power. See picture below.
If you are using a motor block to drive the compressor, instead of an ideal angular velocity source, you won't have the angular velocity directly. In such case, connect an ideal rotational motion sensor in parallel to the compressor, you'll have a measured angular velocity that way.
Another way is to use a "Probe" block in Simscape to extract the power variable from the compressor directly as a Simulink signal.
