Efficiency Calculator — Find η, Energy Input or Energy Output (η = E_out / E_in × 100%)

How to Use the Efficiency Calculator

This energy efficiency calculator is a simple tool for calculating the ratio of useful energy output to the energy input. You can use it to determine the efficiency of heat engines, electric motors, mechanical systems, or any process that converts one form of energy to another.

How to Use

1. Select the unit system (Metric or Imperial/US) and the appropriate energy unit for your values.
2. Choose what you want to find: Efficiency (η), Energy Output (E_out), or Energy Input (E_in).
3. Enter the two known values.
4. Click Calculate — the missing value is computed instantly.

What Is Efficiency?

Efficiency is defined as the ratio of useful energy output to the total energy input. Every time energy is supplied to a machine — for example, to a car engine or an electric motor — a certain portion is wasted as heat, friction, or other losses. Only part of it is converted into the desired useful output. The more efficient the machine, the greater the output it produces for a given input.

How to Calculate Efficiency

The efficiency formula is:

Understanding Efficiency Values

• 0% — All input energy is wasted; zero useful output.
• 100% — No energy is lost; all input is converted to useful output (theoretical ideal).
• >100% — Not physically possible for isolated systems due to energy conservation. (Apparent values over 100% in systems like heat pumps indicate additional heat absorbed from the environment is not measured.)

Energy Units Supported

Metric: Joule (J), Kilojoule (kJ), Megajoule (MJ), Gigajoule (GJ), Kilowatt-hour (kWh), Megawatt-hour (MWh), Calorie (cal), Kilocalorie (kcal).

Imperial (US): British Thermal Unit (BTU), Foot-pound force (ft·lbf), Horsepower-hour (hp·h), Therm (US).

Real-Life Applications

• Internal combustion engines: A gasoline car engine typically has an efficiency of 20–40%. Most of the fuel energy is lost as heat.
• Electric motors: Modern electric motors can reach 90–97% efficiency.
• Power plants: Coal-fired plants are around 33–45% efficient; combined-cycle gas plants reach ~60%.
• LED lighting: LEDs convert 40–50% of electrical energy into visible light, compared to ~5% for incandescent bulbs.
• Solar panels: Typical photovoltaic panels have 15–22% efficiency in converting sunlight into electricity.
• Return on Investment (ROI): By analogy, ROI measures how "efficiently" money is used in an investment.
• Fuel efficiency: Miles per gallon (MPG) describes how efficiently an engine converts fuel into motion.

Frequently Asked Questions

How do I calculate the efficiency of a machine?

1. Determine the energy supplied to the machine (E_in).
2. Measure the useful energy produced by the machine (E_out).
3. Divide E_out by E_in and multiply by 100 to get the percentage.

What is the unit of efficiency?

Efficiency is a dimensionless (unitless) quantity. It is the ratio of two energy values, so the units cancel out. It is typically expressed as a percentage (%).

Can efficiency be greater than 100%?

For a simple isolated system, no — that would violate the law of conservation of energy. However, apparent efficiencies above 100% can occur in heat pump systems because they move heat from the surroundings in addition to the electrical energy supplied. In those cases, the "extra" energy comes from the environment, not from nothing.

What is the most efficient machine?

Electric motors are among the most energy-efficient machines, with modern units reaching 95–99%. In contrast, incandescent light bulbs have an efficiency of only about 5% for visible light production.