Efficiency Calculator

By Bogna Szyk
Last updated: May 05, 2018

This energy efficiency calculator is a simple tool for calculating the ratio of useful energy output to the energy input. You can use it for determining the proportions of heat energy, electric power, mechanical work, or even chemical energy. Continue reading to learn how to calculate efficiency in each of these cases and discover what the real-life applications of the efficiency formula are.

What is efficiency?

Efficiency is defined as the ratio of energy output to energy input. Every time that you supply energy or heat to a machine (for example to a car engine), a certain part of this energy is wasted, and only some is converted to actual work output. The more efficient the machine, the higher output it produces.

A special type of efficiency is the Carnot efficency. It is defined as the efficiency of the Carnot engine, which is an ideal engine that maximizes the energy output.

How to calculate efficiency?

In order to calculate efficiency, you need to apply the following formula:

η = Eout / Ein * 100%


  • η is the efficiency (expressed as a percentage),
  • Eout is the energy output (in Joules), and
  • Ein is the energy input (also in Joules).

The result will be a number between 0% and 100%. An efficiency equal to 0% means that all of the energy is wasted, and the energy output is equal to zero. On the other hand, an efficiency of 100% means that there is no waste of energy whatsoever.

The fundamental law of energy conservation states that you cannot create energy. What follows is that the efficiency of any machine can never exceed 100%. Nevertheless, you will probably come across articles saying that LED lights or heat pumps can have an efficiency of 300% or more.

How is it possible? The apparent efficiency of 300% results from the definition of efficiency that we use. The electrical power supplied to LED lights might be actually lower than the output, but it doesn't mean that energy was created in the process. It merely means that the lights have received some heat energy from the surroundings and converted it into the output energy. As we aren't able to measure this additional input, the apparent efficiency rises above 100%.

Real-life applications

Even though you probably don't notice it, we apply the definition of efficiency to other real-life phenomena. Some examples include:

  • Return On Investment (ROI). If you look at the ROI formula more closely, you will see that it's analogical to the efficiency equation. This value describes what the "efficiency" of your money on an investment is. Unlike energy efficiency, the ROI metric can (and in fact should) exceed 100%.

  • Fuel efficiency. Even though the formula for MPG (miles per gallon) is not directly related to the efficiency equation, it describes how efficiently does your engine convert fuel to actual power. The less efficient the engine is, the more fuel it needs to cover the same distance.

Bogna Szyk
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