How to Calculate Power Efficiency

This calculator works out the power efficiency of any system that converts input power into useful output power, such as an electric motor, a power supply, or an amplifier. You enter the input power and the useful output power, and the calculator returns the efficiency as a percentage, along with the power lost as heat. The formula is straightforward, but getting the units and the meaning of “output” right is where most mistakes happen.

Formula

Efficiency (η) is the ratio of useful output power to total input power, expressed as a percentage:

η = (P_out / P_in) × 100%

Where:

• P_in = total electrical (or mechanical) power supplied to the system, in watts (W).
• P_out = useful power delivered by the system, in watts (W). This is the power that actually does the desired work, not the total power that comes out (which may include heat, light, or sound that is not useful).
• η = efficiency, a dimensionless number usually given as a percentage (%).

The power lost (usually as heat) is simply the difference:

P_lost = P_in − P_out

Worked Example

Using the default values from the calculator: input power = 100 W, output power = 85 W.

1. Write down the formula: η = (P_out / P_in) × 100%
2. Plug in the numbers: η = (85 / 100) × 100%
3. Divide: 85 / 100 = 0.85
4. Multiply by 100: 0.85 × 100 = 85%
5. Power lost: 100 − 85 = 15 W

So the efficiency is 85%, and the system dissipates 15 W as heat. This matches the calculator output.

When to Use This Calculation

Use this whenever you need to know how well a device converts the energy you put into it into the energy you actually want. Common applications:

• Electric motors: input is electrical power (volts × amps), output is mechanical power (torque × angular speed).
• Power supplies and adapters: input is AC wall power, output is DC power at the rated voltage and current.
• Amplifiers: input is DC supply power, output is audio power delivered to speakers.
• LEDs and light bulbs: input is electrical power, output is visible light power (though this is rarely measured directly in practice).
• Engines and turbines: input is fuel chemical power, output is mechanical shaft power.

The result tells you how much of your input energy is being used productively. A higher percentage means a more efficient system, which usually means lower operating cost and less waste heat. The remaining percentage is lost, almost always as heat, which may require cooling.

Common Mistakes

• Using total output power instead of useful output power. A motor produces mechanical power and also waste heat. Only the mechanical power counts as output. If you include the heat, you will get an efficiency close to 100% even for a poor motor.
• Mixing up input and output. Efficiency is always output divided by input. Reversing them gives a number greater than 1 (or 100%), which is physically impossible for a real device.
• Forgetting to convert units. If input is in kilowatts and output in watts, you must convert to the same unit first. For example, 1.5 kW = 1500 W.
• Using peak or rated values instead of actual operating values. A power supply rated at 500 W may only draw 200 W at light load. Use the actual measured input and output at the operating point you care about.
• Rounding too early. If you round the ratio to 0.8 before multiplying, you get 80% instead of 80.5%. Keep at least two decimal places until the final step.

Tips

• When measuring input power for electrical devices, use a wattmeter rather than multiplying voltage and current, because power factor and harmonics can make the simple product inaccurate.
• For mechanical output, measure torque and rotational speed. Power (W) = torque (N·m) × angular speed (rad/s). Convert rpm to rad/s by multiplying by 2π/60.
• Efficiency often varies with load. A motor might be 85% efficient at full load but only 60% at 10% load. Test at the load you actually use.
• If you only know the efficiency and the input power, you can find output power by rearranging: P_out = η × P_in / 100.
• For systems with multiple stages (e.g., a gearbox attached to a motor), multiply the efficiencies of each stage to get the overall efficiency. For example, 0.85 × 0.90 = 0.765, or 76.5%.

Troubleshooting

Efficiency comes out above 100%: You have likely swapped input and output, or you are counting energy that is not actually being input (like ambient heat absorbed). Recheck your measurements. For a passive device like a transformer, efficiency can approach 100% but never exceed it. For a heat pump, the “efficiency” is called COP and can exceed 100% because it moves heat rather than converting it, but that is a different metric.

Efficiency seems very low (e.g., 5%): Check that you are measuring output at the right point. For example, an LED’s output power is the light emitted, not the electrical power going to the LED. Also check for unit errors: if output is in milliwatts and input in watts, you will get a tiny number. Convert everything to the same unit.

Your measurements fluctuate: Efficiency can vary with temperature, voltage, and load. Take several readings and average them. Ensure the device has reached steady operating temperature before measuring.

You don’t know the output power: Some devices (like a simple resistor heater) have an efficiency of essentially 100% because all electrical power becomes heat, which is the desired output. For others, you may need a dynamometer (for motors), a light meter (for lamps), or acoustic measurement (for speakers). If you cannot measure output directly, look up the manufacturer’s datasheet for a typical efficiency value.

Frequently Asked Questions

Can efficiency be more than 100%? No, for any real device that converts energy, the useful output cannot exceed the total input. Values above 100% indicate a measurement error or a misunderstanding of what counts as input and output. (Heat pumps are an exception because they move heat rather than convert it, but that is called COP, not efficiency.)

What is the difference between efficiency and power factor? Efficiency compares useful output power to total input power (both in watts). Power factor is a measure of how effectively AC current is converted into real power, and it only applies to AC circuits. A device can have a low power factor but still be highly efficient at converting the real power it does draw.

How do I calculate efficiency if I only know voltage and current? For DC, input power = voltage × current. For AC, you need to also know the power factor: real power (W) = voltage × current × power factor. Then use that real power as the input in the efficiency formula.

Does efficiency change with load? Yes, almost always. Most devices have a peak efficiency at a certain load, often around 50–75% of full load. Efficiency drops at very light loads (because fixed losses like standby power dominate) and often drops slightly at full load (due to increased resistance losses).

What does an efficiency of 0% mean? It means the device produces no useful output at all; all input power is wasted, usually as heat. This happens, for example, if a motor is stalled (locked rotor) or if a power supply is short-circuited.

Is higher efficiency always better? Usually yes, because it saves energy and reduces heat. However, higher efficiency components often cost more initially. You need to weigh the upfront cost against the long-term energy savings, which depends on how often and how long the device runs.

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in p_in | Input power | W | 100 | 0
in p_out | Useful output power | W | 85 | 0
out eff | Efficiency | % | (p_out / p_in) * 100 | 2
out loss | Power lost as heat | W | p_in - p_out | 2