Energy, power and the units that measure them

By the end you will be able to

• Name the six main forms of energy and trace a real process as a chain of conversions
• Convert between joules, calories, kilowatt-hours, barrels and tonnes of oil equivalent, and move confidently between prefixes from kilo to exa
• Distinguish energy from power, and installed capacity from delivered energy, using the capacity factor
• Compare fuels by energy density per kilogram and per litre and explain what that means for aviation, shipping and gas trade
• Put world energy use of 592 EJ in 2024 into per-person terms for India, China, the EU and the US
• Explain why grids run at high voltage and why nuclear fuel is so concentrated

Contents

1. Part 1 What energy is

   Energy is easiest to understand not as a substance but as a capacity to make things change: to move, warm, light or rearrange matter. Physicists recognise a small number of forms, and almost everything interesting in the world, from a growing rice plant to a steel mill, is energy being passed from one form to another. Learning to see those conversion chains is the first habit of energy analysis.

   • Energy is the capacity to change things, and it comes in a handful of forms
   • Every process is a chain of conversions
   • Stores and flows: savings versus income
2. Part 2 Units and orders of magnitude

   Energy cannot be discussed sensibly without numbers, and the numbers span a staggering range, from the joule needed to lift an apple to the hundreds of exajoules the world uses each year. The units look like a jumble, but they all convert into one another with a handful of factors. Being fluent in those factors, and in steps of a thousand, protects against the most common errors in energy debates.

   • The joule is small, so energy accounting uses bigger units
   • Prefixes climb in steps of a thousand, and mixing them up is the costliest mistake
   • The world used 592 exajoules in 2024, and the gap between countries is tenfold
3. Part 3 Energy versus power

   Energy is an amount. Power is the rate at which that amount flows. The two are linked by time, and confusing them causes more muddled headlines than any other mistake in the field. Getting the distinction right is what lets you turn a claim about gigawatts of new capacity into an estimate of how much electricity will actually be produced.

   • Power is the rate; energy is the amount
   • Capacity is not generation: the capacity factor
   • From a human body to a power station: the scales of power
4. Part 4 Energy in matter: fuels and energy density

   Fuels are stores of chemical energy, and they differ greatly in how much they hold per kilogram and per litre. Those differences are not academic. They explain why aircraft still fly on kerosene, why gas needs pipelines or liquefaction plants, why the world moved from wood to coal to oil and gas, and why nuclear fuel sits in another league altogether.

   • Fuels store energy in chemical bonds
   • Energy density by mass decides what can fly
   • By volume, gas is a thousand times thinner than oil
5. Part 5 Electricity and the nucleus

   Electricity is the most flexible form of energy, able to run a motor, a light, a computer or a furnace with little fuss. Nuclear energy is the most concentrated, packing millions of times more energy per kilogram than any chemical fuel. Both rest on a few physical principles, and those principles explain the shape of modern power systems.

   • Electricity is a carrier, and high voltage keeps losses down
   • Induction, transformers and alternating current built the grid
   • Nuclear energy is a million times more concentrated than chemical fuel
6. Wrap-up Key terms, try it in the table, final quiz, sources