Where electricity is inexpensive or operating time is low, it may take several years for the savings from installation of high efficiency motors to outweigh the difference in initial cost. Furthermore, the economic advantages of energy efficient motors over rewound motors often provide the opportunity for an upgrade to energy efficient motors when old motors burn out. NEMA requires that the average nominal efficiency test method be listed on the nameplate. IEWC 34-2 and JEC 37 test methods result in slightly higher efficiencies than the IEEE 112 method. Current NEMA codes require that the motor nameplate carry both the efficiency and test standards.
Motors perform best at full load.
An underloaded motor, energy efficient or not, is less efficient than a fully loaded motor.