Everybody knows more or less intuitively what "energy" means, but it is not so easy to give a definition for it because it is not directly accessible to our senses: we can only feel its effects when it appears as heat or work. Energy is the property of a physical system that is capable of producing work or an action. This could be the production of mechanical force (energy to get a car or a train moving) or of heat (energy for cooking a meal or heating up the bath water). The amount of energy produced depends on the intensity of the work or the "power" and the time over which the power is supplied.

At the end of the XVIIth century, the French Denis Papin managed to use heat to move his "fardier", but it is only in the second half of the following century that James Watt developed efficient steam engines. During the XIXth century, the understanding of the nature of energy became more and more accurate, and the "laws of thermodynamics" which govern it were formulated: energy conservation when an isolated system changes its state, decay of mechanical energy into heat. It is not before the beginning of the XXth century that Albert Einstein will teach us that energy and mass are two aspects of the same fundamental property of matter.

In daily life, one encounters and one uses energy under many various forms:

- Mechanical Energy powers motors and muscles
- Calorific Energy is used for heating and cooking
- Kinetic Energy is carried by a moving car, a tennis ball or the wind
- Chemical Energy is extracted from our food or the gas tank
- Radiant Energy comes from the Sun or is carried by lasers
- Nuclear Energy is produced in the stars and in nuclear power plants

Under those different forms, it is indeed the same entity. We can, therefore, transform one into another and we use the same units to measure them.

For instance, the chemical energy of the gas-air mixture is transformed into calorific energy in the cylinder of a car motor, then into mechanical energy of the piston, then transferred to the wheels and finally transformed into the kinetic energy of the car. A small part of the mechanical energy is transformed into electrical energy in the alternator and in radiant energy in the headlights.

When heat is transformed into mechanical energy, the efficiency of this transformation depends upon the difference in temperature between a hot source (i.e. steam produced in a boiler or hot gases produced by the combustion of gas in the cylinder of a car motor) and a cold source (river water or outside air). This relationship was formulated in 1824 by Sadi Carnot. This theoretical efficiency would amount to 50% with a steam temperature of 320°C and a river temperature of 20°C (typical values in a nuclear power plant), but the actual efficiency is closer to 35%.
The basic physical units were chosen in relation to the human perception of Nature. A child measures around one metre, and we understand easily what is one millimetre or one kilometre. Similarly, one kilogram is a handy yardstick and anybody can envision one gram or one tonne. Even if it is kind of short, one second means easily something. One newton is the force that gravity exerts on one kilogram: no problem. In an anthill, the unit of length would probably be the millimetre and elephants would evaluate mass by the tonnes…

Conversely, some of the secondary units may be uneasy to handle: one pascal, which measures the pressure exerted by one newton on one square metre, is so small than one needs 100 000 of them to measure the atmospheric pressure: one must use only multiples of this unit (and the becquerel is much worse). A convenient unit is one which is currently used between one thousandth (milli-unit) and one thousand (kilo-unit). We therefore choose units according to the order of magnitude involved.

Scientists use mostly two units to measure energy. For day-to-day physics, one uses the joule (J), a smallish unit which measures the amount of work needed to raise one kilogram over 10 centimetres. At the atomic scale, the energy unit is the electronvolt (eV), which is of the order of magnitude of the energy transfers among molecules during chemical reactions. The power which produces one joule in one second is called watt (W). As one joule is often too small, one uses daily the kilowatt-hour (kWh), which equals 3.6 millions joules (This unit appears on your electricity bill).

The kilowatt-hour is still too small to be conveniently used by the economists: they invented the toe, tonne of oil equivalent. 16 toe represents the average annual energy consumption of one inhabitant of our planet Earth. To measure the yearly consumption of one country, one uses the million toe (Mtoe) or even the billion toe (Gtoe).

One usually recognizes three energy "levels", depending upon the degree to which it has been transformed:
Primary energy is the energy which can be gathered directly from Mother Nature: crude oil from the well, coal from the mine, hydropower from the dam, sunlight on a photocell or heat produced in the core of a nuclear reactor.

Primary energy is seldom used as such. It is usually converted into secondary energy, electricity or fuel, in refineries or power plants. Whether primary or secondary, energy must be transported and distributed to its final consumer: it is then called final energy. Final energy can be the gasoline in your tank, natural gas fed to your cooking stove or electric power from your meter. But even final energy is not what we ultimately need. We need kilometres from our car, heat to cook our food or lighting in our rooms: that is useful energy. As some energy is spent during the conversion, transport and distribution stages, final energy is but a fraction of primary energy. Worldwide, all forms of energy included, final energy amounts to slightly more than half the primary energy.

In rough figures, what are the quantities of energy needed to generate one kWh of electricity?

- Hydropower: 10 tonnes of water falling from 40 metres high
- Windpower: 20 000 m3 of air moving at 60 km/h
- Chemical: combustion of 0.1 kg of fuel
- Biological: one fine French dinner
- Thermal: vaporisation of 1.5 kg of water
- Nuclear: fission of 0.1 milligram of uranium

Our remote ancestors could only use as energy sources the heat radiated by the Sun and the calories of their (raw) food. About 400.000 years ago, Man learned to master the fire and to use fuelwood for heating, barbecuing and meagre lighting. Despite some production by watermills and windmills, wood remained our main energy source till the middle of the XIXth century. With the industrial revolution, coal became prominent while petroleum was mostly used as a substitute to whale oil for lighting. During the second half of the XXth century, Oil took the crown, followed by natural gas, while the first exploitation of nuclear power and modern renewable energy sources was starting. Fuelled with these more and more diverse energy sources, mankind experienced a phenomenal population growth, together with a fantastic, though very unequal, economic development. Now only, we begin to assess to which extent our energy bulimia is affecting our planet.

On distinguishes two wide categories of energy sources: flux-based or renewable sources and stock-based, non renewable sources, fossil and fissile.

Flux-based energy sources are continuously renewed as far as the eye can see. Their main source is the Sun which is predicted to keep heating and lighting our planet for a few billion years. From the Sun comes solar energy (of course) but also biomass, wind power and hydropower via photosynthesis, differences in atmospheric pressure and the evaporation/condensation cycle of water.

Tidal power comes more from the Moon’s attraction and geothermal energy is produced by decay of the radioactive elements of Earth.

Biomass in only renewable below a certain exploitation rate (a higher rate would cause deforestation). Furthermore, if sunlight, winds and tides are renewable and free of charge, the facilities which gather those energies have a limited lifetime, from a few decades for a solar cell or a wind turbine to one century for a dam. They must be built, maintained and then replaced at the end of their useful life. Renewable sources are often intermittent and of low power density. An intermittent source, especially if it is of random force, is not very convenient to produce electrical power because one cannot easily store electricity in large amount. A low power density means large capitation facilities and therefore large quantities of building materials.

As shown on the table below, most of the energy consumed in the world comes from non renewable stocks of fossil fuels, oil, coal and gas, and of fissile materials, uranium and thorium. As a matter of fact, planet Earth keeps fabricating coal and hydrocarbons, but at a rhythm so low as to be negligible: we burn every year what nature took one million years to produce. Indeed, the evaluation of recoverable resources depends from many factors. Exploring farther and deeper with improved exploration and extraction technologies, we shall be able to exploit lower grade deposits but at a higher cost and with a more important impact on our environment. At the end stocks depletion is unavoidable.

Fissile materials are not renewable either, but they represent huge stocks at the human scale. The same can be said of lithium, the feedstock of nuclear fusion.

Stock-based energy sources have a high power density and produce power when needed.