Learn energy scales, units (joules, kWh, toe), global consumption, and conversion losses. Complete guide to understanding primary vs final energy impact.
Introduction: What is Energy?
The term energy comes from ancient Greek “energeia”, which literally means “force in action”. This ancestral definition already reveals the Greeks’ intuition: energy is the combination of force and movement.
Today, the scientific definition of energy characterizes it as a value that describes the state of a physical system and remains constant according to the law of energy conservation. Energy is never lost, it transforms. When one system interacts with another, energy allows the state of that system to be modified.
Different Forms of Energy
Energy manifests itself in several forms in our environment:
Thermal energy: In a forge, transferring thermal energy to a metal piece increases its temperature and allows its shape to be modified.
Potential energy: In a hydroelectric dam, water located at height possesses potential energy. By opening the gates, this energy is released when the water changes altitude to drive the turbines.
Kinetic energy: Your car converts the chemical energy of fuel into kinetic energy to increase its speed and change your position.
Chemical energy: Wood combustion transforms its chemical composition by releasing heat and light.
Nuclear energy: The fission of a uranium nucleus by neutron bombardment modifies its atomic composition while releasing a colossal amount of energy.
The History of Energy: A Quest for Energy Density
Human history is intimately linked to its mastery of energy. This evolution reveals one constant: the permanent search for ever-higher energy density.
Timeline of Energy Mastery
500,000 years ago: Humanity masters fire, its first controlled chemical energy source.
3000 BC: Development of primitive renewable energies – sun, water, and wind – for various applications.
Early 19th century: Coal gradually replaces wood for domestic heating, thus saving many European forests from deforestation.
Mid-19th century: The appearance of steam engines running on petroleum marks a major turning point in industrialization.
Late 20th century: Nuclear energy becomes an important source of electricity production in many countries.
This evolution has led humanity to live today in a world of abundant energy, considerably increasing its capacity to modify its environment.
Energy Measurement Units: How to Quantify?
To understand energy scales, it is essential to master the measurement units used.
The Joule (J): The International Unit
The joule is the official unit of the International System (SI) for measuring energy. In France, average consumption per inhabitant amounts to 180 gigajoules per year (180,000,000,000 joules).
However, this unit remains abstract for most people. What does one joule actually represent?
The Kilowatt-hour (kWh): The Practical Unit
Engineers prefer to use the kilowatt-hour (kWh), much more meaningful for the general public.
Concrete example: If you vacuum for 1 hour with a 1,000-watt (1 kW) appliance, you consume exactly 1 kWh, or 3.6 megajoules.
This unit makes it easy to visualize your daily energy consumption.
Ton of Oil Equivalent (toe)
Economists and energy specialists often handle very large quantities of energy. They therefore use the ton of oil equivalent (toe), which corresponds to the energy released by burning one ton of oil, or 11,600 kWh.
With this unit, each French person consumes on average 4.3 toe per year.
Energy Scales: Global Population and Energy Consumption
Population and Energy Explosion
Analysis of world population evolution reveals a striking fact: in 10,000 years, the population has been multiplied by 1,000. But the breaking point occurred in the mid-19th century with the advent of steam engines and petroleum.
In just 200 years, the world population has been multiplied by 7, while energy consumption has exploded by a factor of 30.
An Alarming Finding
This difference means that each person on Earth now has four times more energy than 200 years ago, and therefore a capacity four times greater to modify their environment.
Primary Energy vs Final Energy: Conversion Losses
A fundamental concept to understand: in nature, we never find electricity, light, or refined petroleum directly.
What is Primary Energy?
Primary energy refers to raw energy available in nature:
- Coal
- Crude oil
- Natural gas
- Uranium
- Renewable energies (solar, wind, hydraulic)
This energy must then be converted into final energy, the one we actually use.
Conversion Rates by Energy Source
Losses during conversion vary considerably depending on the source:
Coal: Only 25% of primary coal is used as coke in the steel industry. The rest is converted into steam to produce electricity, with significant losses.
Crude oil: Excellent conversion rate of 90% into refined petroleum products, thanks to its very practical liquid form.
Natural gas: High conversion rate of 80% into usable purified gas. A portion also serves to produce electricity via gas turbines.
Nuclear energy: Electricity production by thermal means (nuclear, coal, gas) only allows recovery of 30% of primary energy as final electricity. This limit is imposed by the laws of thermodynamics and cannot be significantly improved.
Renewable energies: Hydroelectricity presents excellent conversion efficiencies into electricity. Other renewable sources mainly produce heat.
The 2016 Global Balance
In 2016, global primary energy production amounted to 13,700 million tons of oil equivalent (Mtoe). After conversion to final energy, 30% of this energy was lost.
A notable phenomenon: while oil represents 32% of primary energy, it constitutes 41% of final energy. Why? Because other sources (coal, nuclear, hydraulic) are mainly used to produce electricity, with significant losses.
The Global Energy Mix: Current State
The Persistent Dominance of Fossil Fuels
Despite climate challenges, fossil fuels still represent 80% of global primary energy supply. This proportion has hardly changed over the past 45 years, even though total production has been multiplied by 2.5.
Coal: A Present Energy, Not a Past One
Contrary to popular belief, coal is not an energy of the past. It currently represents nearly 40% of global electricity production.
This reality has important implications. For example, using an electric car in China, where electricity mainly comes from coal, offers little environmental advantage compared to an efficient combustion car. On the other hand, in France where 92% of electricity is decarbonized, the balance is radically different.
Final Energy vs Useful Energy: The Double Loss
The energy process actually involves two conversion stages:
- Primary energy → Final energy: Losses of 10 to 80%
- Final energy → Useful energy: Losses that can reach 80%
Useful energy is what actually produces a desired effect: movement, light, chemical reaction, or heat.
Global Optimization Principle
When designing an energy system, it is crucial to optimize the entire chain, from primary source to final use, rather than focusing on just one stage.
Concrete Energy Scales: Understanding Your Consumption
The Energy Consumption of an Average French Person
A French person consumes on average 47,000 kWh per year, or approximately 130 kWh per day.
Lighting: An Overestimated Impact
Annual lighting of a dwelling represents approximately 450 kWh, or only 1% of a French person’s total consumption.
Even by halving your lighting consumption with LED bulbs, the overall saving will only be 0.5%. Although positive, the impact remains limited.
Transatlantic Flights: A Considerable Impact
A round-trip flight between Europe and the United States consumes 6,000 kWh per passenger, or 13% of the annual energy budget.
Reducing a single transatlantic flight per year has 26 times more impact than changing all your bulbs to LEDs.
The Car: The Major Category
Driving 15,000 km per year by car consumes approximately 15,000 kWh (simple rule: 1 kWh per km), or 32% of the annual energy budget.
Daily carpooling cuts this category in half and represents savings far greater than all small domestic actions combined.
Human Energy vs Fossil Fuels
What a Human Being Can Accomplish
A person at rest consumes 2.5 kWh per day simply to maintain vital functions.
Climbing a 3,000-meter mountain represents considerable effort that produces 0.5 kWh of useful energy.
Lifting 18 tons of rocks to 1 meter height generates only 0.05 kWh of useful energy – exhausting work for very little energy.
The Power of Petroleum
1 liter of petroleum contains 10 kWh of energy and costs approximately 1.50 euros.
For this minimal price, you get the energy equivalent of:
- 20 people climbing a 3,000 m mountain
- 200 people lifting rocks all day
This comparison illustrates why energy is extraordinarily cheap today.
Energy Density: A Decisive Criterion
To produce 0.5 kWh of useful energy (the equivalent of climbing a 3,000 m mountain), you need:
- Lift 1 ton of water to 200 meters height (hydraulic energy)
- Burn 50 grams of petroleum (chemical energy)
- Fission 0.5 microgram of uranium (nuclear energy)
This comparison reveals the immense difference in energy density between sources. Uranium has an energy density 100 million times higher than that of water.
This is why humanity has always sought to use increasingly dense energy sources.
Conclusion: Key Takeaways
Principle #1: No Energy is Inherently “Clean”
In nature, we find only primary energy. The final energy we use is never cleaner than the primary energy used to produce it.
There is no “green” or “clean” energy per se. The dose always makes the poison. If we burn all forests to produce biomass, no one will call this energy clean.
Principle #2: All Energy Use Modifies the Environment
Using energy inevitably means modifying your environment, whether we want it or not. Humanity has never had such abundant and cheap energy to transform the planet.
Principle #3: Fossils Dominate Electricity Production
Two-thirds of global electricity still comes from fossil fuels. The energy transition therefore remains a major challenge for decades to come.
Principle #4: Prioritize High-Impact Actions
Rather than focusing on multiple small actions (LED lighting, economical faucets, etc.), it is much more effective to target major consumption categories:
- Reduce air travel
- Practice carpooling
- Optimize car usage
- Improve home insulation
Going Further
Understanding energy scales is essential for making informed decisions, both individually and collectively. This knowledge allows you to:
- Assess the real impact of our consumption choices
- Identify priority action levers to reduce your energy footprint
- Understand the stakes of national and international energy policies
- Avoid greenwashing by distinguishing symbolic actions from significant transformations
Energy shapes our civilization. Mastering it intellectually means giving ourselves the means to act effectively for a sustainable future.
Keywords: primary energy, final energy, energy scales, energy consumption, kilowatt-hour, energy density, energy transition, energy efficiency, global energy mix, environmental impact energy
Tags: energy, energy scales, consumption, energy efficiency, energy transition, energy mix