Motor Efficiency Formula: How to Calculate It

This calculator works out the efficiency of an electric motor from three values you enter: output power (mechanical), input power (electrical), and optionally power factor if you only know voltage, current, and power factor. You get back the efficiency as a percentage, plus the electrical input power if you entered voltage and current. The formula is simple: efficiency equals output power divided by input power, multiplied by 100.

Formula

Motor efficiency (η) is the ratio of useful mechanical output power to total electrical input power, expressed as a percentage:

η = (P_out / P_in) × 100

Where:

• η = efficiency in percent (%)
• P_out = mechanical output power in watts (W) – this is the power delivered to the load (e.g., a fan, pump, or conveyor)
• P_in = electrical input power in watts (W) – this is the electrical power consumed by the motor

If you don’t know P_in directly, you can calculate it from voltage (V), current (I), and power factor (PF) for AC motors:

P_in = V × I × PF × √3 (for three-phase) or P_in = V × I × PF (for single-phase)

For DC motors, power factor is 1, so P_in = V × I.

Worked Example

Using the defaults from the calculator: output power = 750 W, input power = 900 W.

1. Write down the formula: η = (P_out / P_in) × 100
2. Plug in the values: η = (750 / 900) × 100
3. Divide: 750 / 900 = 0.8333…
4. Multiply by 100: 0.8333 × 100 = 83.33%

The motor efficiency is 83.33%, which matches the calculator output.

When to Use This Formula

Use this formula whenever you need to assess how well a motor converts electrical energy into mechanical work. It is essential for:

• Comparing motors for energy efficiency (higher efficiency means lower running costs)
• Checking if a motor is performing as expected (a drop in efficiency may indicate wear or misalignment)
• Sizing motors for applications – you can determine the required input power given a desired output
• Energy audits and sustainability reporting

How to Read the Result

The efficiency percentage tells you how much of the electrical energy is turned into useful mechanical work. The rest becomes heat, noise, or vibration. For example, 83.33% efficiency means 83.33% of the electrical energy is used for the mechanical output, while 16.67% is lost. Typical electric motors range from about 70% to 96% efficiency, with larger motors usually being more efficient than small ones.

Common Mistakes

• Mixing up input and output power – Output is what you get out (mechanical), input is what you put in (electrical). Swapping them gives a meaningless result.
• Using horsepower without converting – If output is given in horsepower (hp), convert to watts first: 1 hp = 745.7 W.
• Forgetting power factor for AC motors – For AC, input power is not simply V × I; you must multiply by power factor (and √3 for three-phase).
• Using line-to-line vs phase voltage – For three-phase, use line-to-line voltage with the √3 factor, or phase voltage with a factor of 3. Mixing them up causes errors.
• Rounding too early – Keep intermediate values with several decimal places to avoid rounding errors in the final percentage.

Tips

• Always use the same units for both powers – watts (W) or kilowatts (kW). If you use kW, the formula still works because the ratio is dimensionless.
• For a quick estimate, you can measure input power with a wattmeter and output power with a dynamometer, but for many practical purposes, output power is known from the motor nameplate or the driven equipment’s requirement.
• If you know the motor’s rated efficiency (from the nameplate), you can calculate the input power required for a given output: P_in = P_out / (η/100).
• Remember that efficiency varies with load – a motor is most efficient near its rated load, not at light loads.

Troubleshooting

If your calculated efficiency is above 100%: This is physically impossible. Check that you have not swapped output and input. Also verify that your output power is actually the mechanical power, not the electrical power of the load.

If efficiency seems very low (below 50%): The motor might be severely overloaded or underloaded, or there could be a measurement error. Double-check the power factor and voltage values. For small motors, low efficiency is normal at light loads.

If you get a negative efficiency: You likely entered a negative output or input power. Check your inputs – power values should be positive.

If you are using the calculator and the result looks wrong: Make sure you entered the output and input in the same unit (watts). If you are using kilowatts, convert to watts by multiplying by 1000, or use the same unit for both – the ratio will be the same.

Frequently Asked Questions

What is a good efficiency for an electric motor?

Most standard induction motors have efficiencies between 70% and 96%. Larger motors (above 10 kW) tend to be more efficient, often above 90%. Premium efficiency motors can reach 96% or higher.

Can efficiency be more than 100%?

No, that would violate the law of conservation of energy. A motor cannot output more mechanical power than the electrical power it consumes. If you get a value above 100%, there is an error in your measurements or calculations.

How do I measure output power for a real motor?

Output power is usually measured with a dynamometer, which applies a load and measures torque and speed. Then P_out = torque (N·m) × angular speed (rad/s). For many applications, you can also estimate output from the motor nameplate if it is running at rated conditions.

What is the difference between efficiency and power factor?

Efficiency is the ratio of useful mechanical power to electrical input power. Power factor is the ratio of real power (watts) to apparent power (volt-amperes) in an AC circuit. Both affect the actual electrical consumption, but they are different concepts.

Does motor efficiency change with load?

Yes. Motors are most efficient near their rated load (typically 75% to 100% of rated). At very light loads, efficiency drops significantly because fixed losses (like core losses and friction) remain constant. At overload, efficiency also drops due to increased copper losses.

How can I improve motor efficiency?

Use a motor that matches the load (avoid oversizing), maintain proper lubrication and alignment, keep the motor clean, and consider using variable speed drives for applications with varying loads. Replacing an old motor with a high-efficiency model can also help.

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in p_out | Output power (mechanical) | W | 750 | 0
in p_in | Input power (electrical) | W | 900 | 0
out eff | Efficiency | % | (p_out / p_in) * 100 | 2