MOTOR POWER CALCULATOR

Calculate motor power in watts, kilowatts, and horsepower using torque and RPM, or electrical inputs such as voltage, current, power factor, and efficiency. Choose a calculation method below, enter your values, and get input power, output power, and horsepower instantly.

What Is Motor Power?

Motor power is the rate at which a motor converts supplied energy into useful work. For an electric motor, that means converting electrical energy at the terminals into mechanical energy at the rotating shaft. Power is measured in watts (W) or kilowatts (kW), and often expressed in horsepower (HP) for legacy or regional conventions.

Two figures matter when talking about motor power: electrical input power, which is what the motor draws from the supply, and mechanical output power (also called shaft power), which is what actually reaches the load. Every real motor loses some energy internally as heat, so output power is always somewhat lower than input power. The ratio between the two is the motor's efficiency.

Rated motor power — the value printed on a nameplate — is the output power the manufacturer guarantees under specific standard conditions. It is not automatically the same as a value you calculate from field measurements, because your operating voltage, load, and ambient conditions may differ from the test conditions used to rate the motor.

Power matters because it determines what a motor can actually do: how much torque it can sustain at a given speed, how heavy a load it can drive, and how much electrical capacity — wiring, breakers, inverter or VFD sizing — the installation needs to support it.

Motor Power Formulas

The formula you use depends on which quantities you know. The table below summarizes the core relationships used throughout this page and inside the calculator.

P = power, T = torque, ω (omega) = angular velocity in radians per second, V = RMS voltage, I = RMS current, PF = power factor, η (eta) = efficiency as a decimal.

Mechanical Motor Power

Mechanical power is the product of torque and angular velocity: P = T × ω. Angular velocity in radians per second is derived from RPM using ω = RPM × 2π / 60. A motor can reach the same power by producing high torque at low speed, or lower torque at high speed — power is what torque and speed produce together, not either one alone.

Motor Power from Torque and RPM

The constant 9550 comes from combining the 2π/60 conversion with the 1000 W-to-kW conversion. For a more exact mechanical result, use the direct angular-velocity form: P(W) = Torque(N·m) × RPM × 2π / 60.

DC Motor Power

For a DC motor, electrical input power is simply voltage multiplied by current. The mechanical output at the shaft is always lower, because internal losses convert part of that input into heat rather than motion. The main loss mechanisms are:

• Copper losses — resistive heating in the winding, proportional to current squared
• Core losses — hysteresis and eddy currents in the magnetic core
• Mechanical friction — bearing and brush friction
• Windage — aerodynamic drag on rotating parts
• Controller losses — switching and conduction losses in any driver or speed controller

Single-Phase AC Motor Power

In an AC circuit, voltage and current alone only describe apparent power (in volt-amps). Power factor accounts for the phase difference between voltage and current, converting apparent power into real power — the power that actually does work. Multiplying real input power by efficiency gives the estimated mechanical output.

Three-Phase AC Motor Power

The √3 factor (approximately 1.732) accounts for the phase relationship across three conductors. The voltage in this formula must be line-to-line voltage — measured between any two of the three supply lines — not the phase voltage measured between one line and neutral. Using phase voltage in this formula will understate the true power by roughly a factor of √3.

Motor Efficiency

Efficiency describes how much of the electrical energy supplied to a motor is converted into usable mechanical energy at the shaft. A motor that draws 5 kW and delivers 4.5 kW of shaft power is 90% efficient — the remaining 0.5 kW is dissipated as heat.

Efficiency is not a fixed number. It changes with load level, running speed, winding temperature, and how close the motor is operating to its rated duty point. A motor running well below its rated load is often noticeably less efficient than one running near its rated point, and efficiency generally drops off again well above rated load as losses grow faster than output.

Motor Input Power vs Output Power

Output power is normally lower than input power for any real motor, because winding resistance, magnetic losses, and friction all consume part of the supplied energy along the way.

Motor kW to HP Conversion

These are direct mathematical conversions, not standard motor nameplate ratings — nameplate HP values are typically rounded to conventional frame sizes and may not match a raw conversion exactly.

Motor HP to kW Conversion

Motor Power Examples

Fifteen worked examples covering DC, torque/speed, single-phase, and three-phase calculations. Each follows Given → Formula → Calculation → Result. Appliance and load examples below are illustrative only, not universal ratings.

Motor Power Reference Table

Illustrative input-power calculations at a fixed power factor of 0.80, for quick reference only — always calculate your own values above.

How to Choose Motor Power

A calculated running-power figure is a starting point, not a complete motor specification. Real motor selection weighs several factors together:

• Mechanical load and required torque — the torque the driven equipment needs at working speed
• Starting torque — many loads need significantly more torque to start moving than to keep running
• Operating speed — the RPM range the load requires, and any gear ratio between motor and load
• Duty cycle — continuous, intermittent, or short-time duty changes thermal requirements
• Efficiency and power factor — affect running cost and supply sizing, not just shaft output
• Service factor — a margin above rated load the motor can tolerate briefly
• Ambient temperature and altitude — both affect a motor's real thermal capacity
• Load variation — how much the load fluctuates during operation
• Manufacturer specifications — the definitive source for a given motor's actual capability

Because of these factors, motor sizing should never be reduced to picking the next motor size up from a single running-power calculation.

Motor Power for Pumps

Pump motor sizing depends on flow rate, total head, fluid density, and both pump and motor efficiency — not on voltage and current alone. Two pumps drawing the same electrical current can require very different mechanical power depending on what fluid they move and how high they lift it.

Motor Power for Fans

Fan motor power depends on airflow rate, static pressure, rotational speed, torque, and fan efficiency. Two fans producing the same airflow can need different motor ratings depending on the pressure they must work against, which is why the mechanical load — not just nameplate current — sets the required rating.

Motor Power for Compressors

Compressor motor sizing depends on discharge pressure, flow rate, compression ratio, and mechanical and volumetric efficiency. These variables interact, so compressor duty typically needs manufacturer performance curves rather than a single power formula.

Common Motor Power Calculation Mistakes

• Confusing watts (a rate of power) with watt-hours (a quantity of energy)
• Confusing real power (kW) with apparent power (kVA)
• Treating input power and output power as the same value
• Ignoring power factor in AC power calculations
• Ignoring efficiency when estimating output power
• Using the wrong voltage convention for a three-phase system
• Using phase voltage instead of line-to-line voltage in the √3 formula
• Assuming current alone determines motor power
• Assuming RPM alone determines motor power
• Treating horsepower and kilowatts as numerically identical
• Ignoring starting current, which is typically much higher than running current
• Ignoring starting torque requirements of the driven load
• Ignoring the motor nameplate in favor of a purely theoretical calculation
• Treating a single calculated power value as a guaranteed motor size recommendation

Frequently Asked Questions

Related Calculators