Calculate electric motor horsepower from volts, amps, efficiency, power factor, and supply type without mixing up input and shaft power.
Advanced options
Table of contents
How to use our Electrical Horsepower Calculator
- Choose Power supply type: DC, Single-phase AC, or Three-phase AC.
- Enter Voltage (V) and Current (A). For Three-phase AC, use line-to-line voltage and line current.
- Enter Motor efficiency (%) and, for AC only, Power factor.
- Use Advanced options only if you need a different Horsepower standard, Number format, or Digits to show.
- Check whether Estimated shaft horsepower is lower than Electrical input horsepower when efficiency is below 100%; if not, recheck the efficiency and supply type.
Definitions
Power supply type: The kind of electricity feeding the motor: DC, Single-phase AC, or Three-phase AC.
Voltage (V): Electrical push measured in volts. For Three-phase AC, the calculator uses line-to-line voltage.
Current (A): Electrical flow measured in amps. Running current is the best match for normal running horsepower.
Motor efficiency (%): The percent of electrical input power that becomes useful shaft output power.
Power factor: An AC-only number from 0 to 1 that turns volt-amps into real watts.
Estimated shaft horsepower: The estimated useful output horsepower after efficiency losses.
Electrical input horsepower: The real electrical input power converted to horsepower before efficiency losses.
Apparent power: AC volt-amp power before multiplying by power factor, shown in VA.
Horsepower standard: The watts-per-horsepower constant used for the hp conversion, such as electric hp at 746 W per hp [1].
Common mistakes and quick fixes
Mistake: Comparing Electrical input horsepower to a motor output nameplate rating.
Fix: Use Estimated shaft horsepower when you want the estimated useful motor output.
Mistake: Entering starting current in Current (A).
Fix: Use running current for running horsepower; starting current is much higher and will overstate Electrical input power.
Mistake: Choosing Three-phase AC but entering phase-to-neutral Voltage (V).
Fix: Enter the line-to-line Voltage (V) for Three-phase AC.
Mistake: Leaving Power factor blank for Single-phase AC or Three-phase AC.
Fix: Enter a Power factor greater than 0 and no more than 1.
Mistake: Typing Motor efficiency (%) as 0.90 instead of 90.
Fix: Enter Motor efficiency (%) as a percent, such as 90 for 90%.
Mistake: Using Apparent power as real motor power.
Fix: Use Electrical input power for real AC watts; Apparent power is volts times amps before power factor is applied.
Limitations & Key Assumptions / Boundary Conditions
- This calculator estimates power only. It does not size wires, breakers, fuses, overloads, starters, or generators.
- Three-phase results assume balanced power, line-to-line Voltage (V), and line Current (A).
- Power factor is used only for Single-phase AC and Three-phase AC. It is hidden and ignored for DC.
- Motor efficiency (%) and Power factor can change with load. A nameplate or test value is better than a guess.
- Estimated shaft horsepower does not include belt, gearbox, pump, fan, or other mechanical losses after the motor shaft.
- Very low current can produce 0 or near-0 horsepower. That is valid if Current (A) is 0 or almost 0.
- The selected Horsepower standard changes only the hp conversion. Watts and kW do not change.
Methodology
How the power is found
The calculator first finds real electrical input power in watts from the selected Power supply type. DC uses volts times amps. Single-phase AC also multiplies by Power factor. Three-phase AC uses the balanced three-phase factor sqrt(3), line-to-line voltage, line current, and Power factor [2][3].
DC: P_input_W = V * I
Single-phase AC: P_input_W = V * I * PF
Three-phase AC: P_input_W = sqrt(3) * V_LL * I_L * PF
How shaft output is estimated
Motor efficiency lowers the usable shaft power because some input power becomes heat and other losses.
P_shaft_W = P_input_W * (efficiency_percent / 100)
How horsepower is converted
Both input watts and shaft watts are divided by the selected watts-per-horsepower constant. The default electric horsepower constant is 746 W per hp [1].
input_hp = P_input_W / watts_per_hp
shaft_hp = P_shaft_W / watts_per_hp
The calculator also shows Electrical input power in kW by dividing input watts by 1000.
input_kW = P_input_W / 1000
Apparent power for AC
For AC modes, Apparent power shows volt-amps before Power factor is applied. This helps explain why AC volts times amps can be larger than real watts.
Single-phase AC: VA = V * I
Three-phase AC: VA = sqrt(3) * V_LL * I_L
Mini example
For Single-phase AC with Voltage (V) = 230, Current (A) = 10, Power factor = 0.85, and Motor efficiency (%) = 90, input power is 230 * 10 * 0.85 = 1955 W. Shaft power is 1955 * 0.90 = 1759.5 W. Using electric horsepower, Estimated shaft horsepower is 1759.5 / 746 = 2.36 hp, and Electrical input horsepower is 1955 / 746 = 2.62 hp.