What Does Energy Conversion Efficiency Mean?

• All the energy put in
• The useful part
• The part that becomes waste heat

What you will learn

• State how efficiency is worked out from two amounts of energy.
• Explain where the non-useful energy goes.
• Read efficiency in the examples of a car, a power plant and a solar cell.

Before you start

You only need to know that energy comes in many forms, such as heat, motion and electricity. No formulas are needed.

How is efficiency worked out?

Efficiency is the ratio of the useful part to the total energy put in. The energy principles page of the University of Illinois Urbana-Champaign defines it: energy efficiency is the amount of useful energy extracted from a system divided by the total energy put into the system.

Power plants have their own way of measuring. An EIA FAQ page says the heat rate is the amount of energy a plant uses to produce one kilowatt-hour of electricity. Divide the 3,412 Btu in a kWh of electricity by the heat rate in Btu/kWh to obtain the efficiency ratio, then express that ratio as a percentage. For example, if the heat rate is 10,500 Btu, the efficiency is approximately 33% in EIA's example.

• Useful energy above the line
• Total energy put in below the line

Where does the remaining energy go?

It still exists, just no longer useful. The Illinois page recalls the first law: total energy is constant, so all of the energy must end up somewhere, in its original form or a different one. Energy that is not used goes into waste heat, sound, thermal vibrations or light.

In a chain with losses at each step, an extra conversion adds another place for energy to become waste heat. This is how the Illinois page explains a conversion chain; step count alone cannot rank two different devices. A gasoline car is an example: the chemical energy in the fuel becomes heat, and then mechanical motion. The textbook illustration uses an overall efficiency of about 30%, corresponding to 70% of the original fuel energy becoming waste heat. These are values for that illustration, not universal limits for every vehicle or engine.

• Four blocks in a row, each one an energy conversion step
• Waste heat rising at the middle steps
• What is left after each step gets shorter

Where does efficiency show up in daily life?

In every device that changes energy, each with its own losses.

For vehicles, the fueleconomy.gov page says that in gasoline vehicles most of the fuel's energy is lost in the engine, primarily as heat. Hybrids, plug-in hybrids and electric vehicles use regenerative braking to recover some braking energy that would otherwise be lost.

For solar cells, the U.S. Department of Energy page defines conversion efficiency as the percentage of the solar energy shining on a cell that is converted into usable electricity. Not all of the sunlight that reaches a cell becomes electricity.

• Power plant: a chimney
• Vehicle: wheels
• Solar cell: a tilted panel

Real-world example

Think of pouring water from a kettle into a cup through a leaky funnel. The water poured in is the total put in, the water reaching the cup is the useful part, and the water dripping outside is the loss. The leakier the funnel, the lower the efficiency.

The comparison is only a memory aid. Ask yourself: if you pour through two leaky funnels in a row, will more or less water remain than with one? Less, just as more conversion steps lose more heat.

Common misunderstandings

Lost energy does not vanish. It goes somewhere, usually as waste heat.

Whether heat counts as useful depends on the intended purpose. In the motion example, heat released to the surroundings is outside the useful motion output. A ratio needs a defined input and output.

Low efficiency does not mean the device is broken. The textbook's value of about 30% illustrates energy allocation; it is not a test for a broken vehicle.

Frequently asked questions

What efficiency counts as good?

It depends on the device. The power plant in the EIA example reaches approximately 33% at one heat rate, while the Illinois page illustrates a gasoline car with about 30%. These examples do not set a shared target for every device.

Where does lost energy go?

It becomes waste heat, sound, thermal vibrations or light, forms that cannot be used for the original purpose.

How does regenerative braking help?

It recovers some braking energy that would otherwise be lost, so it reduces the loss.

Summary

• Efficiency is useful energy divided by total energy put in.
• The rest becomes waste heat or other unusable forms, outside the useful output being considered.
• In a lossy chain, each conversion is another place to lose useful output.