Planetary gears- efficiency and output torque

I want to use a planetary gear train to reduce the output speed of a motor. Knowing the output (speed and torque) of the motor (which will be the input to the gear train), I want to calculate the output torque and efficiency of the gear train. Not sure yet on what the gear ratio will be and how many planet gears will be used (may be a multi-stage system). Any suggestions would be greatly appreciated.