Electrical power is in general rated in Watts or Horsepower. A horsepower is a unit of power equal to 746 watts or 33,000 lb.ft per minute (or 550 lb.ft per second).
A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt. A watt is 1/746 of 1 horsepower.
Even if the watt is the base unit of electrical power, its common to rate motor power in either horsepower or watts.
Electric power of a motor can be expressed as:
Pw = ηm U I / 1000 (1)
Pw = power (kW)
ηm = motor efficiency
U = voltage (V)
I = current (A, amps)
Pw = ηm U I PF / 1000 (1b)
PF = Power Factor
Pw = ηm 2 U I PF / 1000 (1c)
Pw = ηm 1.73 U I PF / 1000 (1d)
Horse power of a motor can be expressed as:
Php = Pw / 746 (2)
Php = ηm U I / 746 (2b)
Php = horsepower (hp)
The horse power of an 230 V electrical motor with 85% efficiency pulling 10 amps can be calculated as:
Php = 0.85 230 (V) 10 (amps) / 746
= 2.62 (hp)
Standard horsepower ratings of electrical motors - 1 to 4000 hp are indicated below: