The law of conservation of energy
The single most important fact about energy fits in one sentence: energy is neither created nor destroyed. The EIA puts it this way in its page on the laws of energy, and it is one of the foundations of physics. When people use energy, it does not disappear. It changes from one form into another, and the total amount stays the same.
The first guide sorted energy into stored (potential) and moving (kinetic) forms. Conservation is the rule that connects them. A book falling from a shelf loses gravitational potential energy at exactly the rate it gains kinetic energy, if air resistance is ignored. The physics text on LibreTexts used in that guide shows the same balance with a formula: at the start the book's energy is all m g h, partway down it is part m g h and part ½ m v², and just before landing it is all ½ m v². At every moment the two parts add up to the same total.
This is what makes energy so useful as an idea. If you know how much energy went into a system, you know how much must come out, even if you cannot yet see where it has gone. When the numbers do not balance, the missing energy has not vanished; it has gone somewhere that has not been counted, very often into heat.
Two meanings of "conservation"
The word causes confusion because it has two meanings. In everyday speech, "energy conservation" means saving energy: switching off lights or insulating a loft so that less electricity or fuel is bought. The EIA notes that to scientists, energy conservation means something else entirely. It is the law that the total amount of energy does not change.
Both meanings are sensible, and they fit together once the physics is clear. Saving energy at home never destroys any energy. It means converting less of a fuel or of the electricity supply into forms that are hard to use again, such as warm air leaking out of a window. On this site, "conservation" in a physics sentence always means the law.
Everyday conversion chains
Most of the energy people use passes through several forms before it does anything useful. The EIA gives a number of examples across its Energy Explained pages:
- A car engine burns gasoline, converting the chemical energy in the fuel into mechanical energy that turns the wheels.
- A solar photovoltaic cell changes radiant energy from the Sun directly into electrical energy.
- A fireplace converts the chemical energy in wood into thermal energy.
- The human body stores the chemical energy in food and uses it as kinetic energy during work or play.
- A power grid starts with the chemical energy in coal or natural gas, or the kinetic energy of a flowing river, converts it into electrical energy, and then into light and heat in homes.
Every link in such a chain is a conversion, and every conversion obeys the conservation law. What changes from link to link is not the total, but how much of it is in a form that can still do the job people want done.
Efficiency: the useful share
The EIA defines energy efficiency as the amount of useful energy obtained from a system. A perfectly efficient machine would convert all the energy it uses into useful work. No real machine does. Converting one form of energy into another always produces some usable energy and some unusable energy, and many conversions are, in the EIA's words, relatively inefficient.
Efficiency is usually written as a fraction or a percentage:
Efficiency = useful energy out ÷ total energy in
Because energy is conserved, the energy that is not useful still has to be somewhere. In most machines it ends up as heat: the warmth of a running motor, the hot exhaust of an engine. That heat is not lost from the universe. It is simply spread out in a way that makes it hard to collect and use again.
The EIA's own example is the human body. Food gives a person energy to move, breathe and think, but the body is less than 5% efficient at converting food into useful work most of the time. The rest becomes heat, which may or may not be welcome depending on how warm the person wants to be.
Useful depends on the job
Whether energy is "useful" depends on what you want. Heat from a light bulb is waste when the goal is light, and the whole point when the goal is a warm room. Efficiency is always measured against a stated purpose.
Example: an energy budget
Worked example
An illustration with invented round numbers, not a measurement of any real device. A small electric motor is supplied with 1,000 joules of electrical energy and lifts a load, giving it 300 joules of gravitational potential energy.
- Useful energy out is 300 joules; total energy in is 1,000 joules.
- Efficiency is 300 ÷ 1,000 = 0.3, or 30%.
- Conservation says the other 700 joules must still exist. In a motor, most of it would show up as heat in the windings and bearings, with a little as sound.
- Add the parts back together: 300 + 700 = 1,000 joules. The budget balances, as it always must.
The same three-step habit works for any device: name what goes in, name the useful output, and account for the remainder. It is a quick way to see through any claim that a machine puts out more energy than it takes in. Under the conservation law, no device can do that.
Conversion at a power plant
Electricity generation is a long conversion chain, and the EIA's page on how electricity is generated sets it out step by step. In most power plants a moving fluid, such as water, steam, combustion gases or air, pushes a series of blades mounted on a rotor shaft. The force on the blades spins the shaft, and the generator attached to it converts the mechanical energy of the spinning rotor into electrical energy.
Where the fluid is steam, a fuel is burned in a boiler, or nuclear fuel rods produce heat, to make the steam in the first place. So a coal plant converts chemical energy to thermal energy, thermal energy to the motion of steam, the motion of steam to the rotation of a turbine, and that rotation to electricity. Some of the energy at each step leaves as heat that does not become electricity.
The EIA describes one way of recovering some of that heat. Combined heat and power plants use heat that is not converted to electricity for industrial processes or for space and water heating, and the EIA counts them, along with combined-cycle plants that pass hot gases from one turbine on to drive a second, among the most efficient ways to turn a combustible fuel into useful energy. The physics has not changed: the total energy is the same. What has changed is that more of the total is put to use.
Why the accounting matters
Conservation and efficiency explain most of what follows on this site. The electricity guide follows energy through the grid, where some is lost as heat in the wires. The home energy guide uses the same accounting to explain why the energy a lamp or a heater draws from the wall must all end up somewhere in the house. And the food guide returns to the human body as a machine that turns chemical energy into motion and heat.
In each case the rule is the same, and it never has exceptions in everyday life: count the energy in, count the energy out, and the two will match.