Energy Efficiency Calculator

About the Energy Efficiency Calculator

Efficiency measures how much of the energy (or power) put into a machine comes out as useful work. It is the ratio of useful output to total input, expressed as a percentage.

Enter the useful output and total input in the same units (joules, watts, etc.). Efficiency is returned as a percentage; it can never exceed 100%.

How to use this calculator

1. Enter the useful output energy, \(E_{out}\).
2. Enter the total input energy, \(E_{in}\).
3. Use the calculator to find \(\eta\) as a percentage.
4. Check that both energies use the same unit, like joules.

The formula explained

The formula \(\eta = \frac{E_{out}}{E_{in}} \times 100\%\) computes efficiency as the fraction of input energy that is useful output, written as a percent. A higher percentage means less energy was wasted.

• \(\eta\) = efficiency, in percent
• \(E_{out}\) = useful output energy
• \(E_{in}\) = total input energy

Step by step method

1. Divide the useful output energy by the total input energy.
2. Multiply the result by \(100\%\).
3. Write the answer as a percent.
4. Make sure \(E_{out}\) is not greater than \(E_{in}\), because efficiency cannot be more than \(100\%\).

Worked example

Problem. A machine uses \(500\,\text{J}\) of input energy and gives \(125\,\text{J}\) of useful output energy. Find its efficiency.

1. Substitute the values into the formula: \(\eta = \frac{125}{500} \times 100\%\).
2. Compute the fraction: \(\frac{125}{500} = 0.25\).
3. Multiply by \(100\%\): \(0.25 \times 100\% = 25\%\).

Tips and common mistakes

• Always use the same energy units for \(E_{out}\) and \(E_{in}\).
• If your answer is over \(100\%\), check your numbers, because that usually means one value was entered wrong.

Frequently asked questions