Motor Horsepower Calculator
Electric motor power can be expressed in horsepower (HP), kilowatts (kW), or kilovolt-amperes (kVA), and each measure captures a different aspect of motor performance. This calculator converts voltage and current to all three using power factor and motor efficiency. kVA is apparent power: the product of voltage and current, representing what the supply circuit must deliver. kW is real power: apparent power multiplied by power factor, representing what actually flows into the motor windings. HP is shaft output power: real power multiplied by motor efficiency, representing mechanical work delivered to the load. The conversion uses 1 HP = 746 W, the electrical horsepower as defined by NIST and used in NEMA MG 1 motor standards. Default values (power factor 0.85, efficiency 0.90) are representative of a typical NEMA Design B induction motor at full load. NEMA Premium efficiency motors may achieve 91% to 95% efficiency. Adjust the inputs to match your motor's nameplate data for the most accurate result. For three-phase motors, the voltage input should be the line-to-line voltage, and the current should be the line current; the formula includes the square root of 3 factor automatically if you select three-phase.
Motor power formulas
Single-phase: kVA = V x I / 1,000
Three-phase: kVA = V x I x 1.732 / 1,000
Real input power (kW) = kVA x Power Factor
Shaft output (kW) = Real input (kW) x Efficiency
Shaft output (HP) = Shaft output (kW) x 1,000 / 746
Worked example (single-phase)
A single-phase motor at 240 V, 10 A, PF 0.85, efficiency 0.90:
- kVA = 240 x 10 / 1,000 = 2.40 kVA
- Real input power = 2.40 x 0.85 = 2.04 kW
- Shaft output = 2.04 x 0.90 = 1.84 kW
- Shaft HP = 1,836 / 746 = 2.46 HP
Frequently asked questions
What does horsepower mean in an electric motor?
Horsepower (HP) is a unit of power equal to 746 watts (as defined by NIST and IEEE). For electric motors, nameplate HP refers to shaft output power: the mechanical power delivered by the motor to the load. The electrical input power is always higher than the shaft output because some energy is lost to heat in the windings and core. A 1 HP motor drawing 746 W of shaft output power typically draws 800 W to 850 W of electrical input power, depending on efficiency.
Why do power factor and efficiency both matter?
Power factor (PF) accounts for the phase difference between voltage and current in AC circuits. Motors are inductive loads, so current lags voltage. PF is the cosine of this phase angle: a PF of 0.85 means 85% of the apparent power (kVA) is doing useful work (kW). Efficiency accounts for heat losses inside the motor itself: at 90% efficiency, 90% of the real electrical input power (kW) is converted to shaft mechanical output. To get shaft HP, you need to account for both: apparent power x PF gives real input power, then x efficiency gives output power.
What is the difference between FLA (full-load amps) and actual current?
Full-Load Amps (FLA) on a motor nameplate is the current the motor draws when delivering its rated shaft output at rated voltage. Actual current can vary: it is lower at partial load and significantly higher during startup (locked-rotor current, often 6x FLA). For wiring and circuit breaker sizing, the NEC requires calculations based on FLA from NEC Table 430.248 (single-phase) or Table 430.250 (three-phase), not the nameplate FLA. The NEC values may differ from nameplate FLA.
How do I convert HP to kW?
One mechanical horsepower = 745.7 watts = 0.7457 kW. One electrical horsepower = 746 watts exactly (IEEE and NIST definition used for motor ratings). To convert: kW = HP x 0.746. To go the other way: HP = kW / 0.746. Note that metric horsepower (used in some European contexts) = 735.5 W, slightly different from the US/UK definition.
What are NEMA Premium motor efficiency classes?
NEMA (National Electrical Manufacturers Association) defines motor efficiency classes in NEMA MG 1. The Premium Efficiency class (IE3 under IEC 60034-30) represents the highest standard efficiency level required in the US under the Department of Energy's EISA 2007 regulations for motors 1 HP to 500 HP. A 10 HP NEMA Premium motor typically achieves 91% to 92% efficiency at full load. The Energy Independence and Security Act (EISA 2007) mandated that most general-purpose motors meet NEMA Premium standards from 2011.
Sources
- NEMA MG 1-2021: Motors and Generators (PDF scope). Motor efficiency classes and definitions.
- NIST: The International System of Units (SI), SP 330. 1 HP = 746 W (electrical horsepower).
- U.S. Department of Energy, EISA 2007: Electric Motor Efficiency Standards.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Results are based on nameplate data and the ideal motor model. Verify against your motor's performance curves for precise values.