As stated in the EASA literature Understanding Energy Efficient Motors, the No. 1 most common misconception about energy-efficient motors is that "an oversized motor is less efficient". The bottom line is that a motor often runs most efficiently somewhere between 60 and 85 percent full load. This means that a 30-horsepower load would be more efficiently supplied by a partially loaded 50-horsepower motor than a fully loaded 30-horsepower motor.
For more information on making motor replacement decisions using energy analysis software, go to: http://www.pdma.com/webinars/MCEGold2.2/EnergyCostanalysis.html.
To get a copy of EASA literature on energy efficiency, go to http://www.easa.com.