How much horsepower does the motor need?
A motor of 1/3 HP is enough for doors in the 8 ft (2440 mm) high to 12 ft (3660 mm )wide range. Use a 1/2 HP motor for doors measuring over 12 ft. (3660 mm). Keep in mind that an electric garage door opener is only meant to work as a substitute for human strength. Therefore, the door must be well-balanced at all times in order to maintain its dead weight at 3 to 4 kilograms.
Consult : www.liftmaster.com