How do I Determine Exhaust Fan Horsepower Based on Electrical Data?
Determining the horsepower rating of an exhaust fan is quite easy provided you have the proper information. You'll need the amperage and the voltage of the exhaust fan or the wattage to calculate horsepower using this method.

Determine the amperage of the exhaust fan. The fan should have a data plate with the manufacturer's name, the UL certification, and electrical requirements; look for the amperage here.

Determine the fan voltage. Most household exhaust fans require 120 volts, but industrial fans may operate at higher voltages. Check the data plate for the proper rating.

Multiply the voltage and the ampere rating to determine the motor wattage.
For instance, 120 volts times 8 amps equals 960 watts.

Convert wattage to horsepower. Multiply the wattage by 0.001341 or divide the wattage by 746. For instance, 960 watts times 0.001341 equals 1.3 horsepower, or 960 watts divided by 746 equals 1.3 horsepower.

Correct for motor efficiency. No motor is 100 percent efficient. For our purposes, it is safe to assume 80 percent efficiency, so multiply your horsepower figure by 0.8. For example, 1.3 horsepower time 0.8 equals 1.0 horsepower.
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Tips
 Many motor data plates state wattage, allowing you to skip to step four.
 To determine the horsepower of three phase motors, multiply the wattage by 1.73, then calculate as for a single phase motor.