Where our energy comes from

Hold a litre of petrol in your hand. It weighs about three-quarters of a kilogram — less than a full water bottle. Inside that small, light, pourable liquid sits enough energy to drive a car ten kilometres or more. Now picture the battery that would store the same energy. It is the size of a suitcase and weighs more than you do. Same energy, wildly different package. That gap — energy packed tight and light versus energy stored loose and heavy — is the single biggest reason the world runs on fossil fuels. And it is the reason changing that is so hard.

Fossil fuels are ancient sunlight in a tin

Start with what they actually are. Coal, oil, and natural gas are not minerals that were always in the ground. They are the buried remains of living things — and those living things ran on sunlight.

Hundreds of millions of years ago, plants and tiny ocean plankton captured solar energy through photosynthesis, the process that turns light, water, and carbon dioxide into the chemical energy of living tissue. They died. Some sank into swamps and seabeds before they could fully rot, and got buried under layer after layer of sediment. Over vast stretches of time, heat and pressure squeezed and cooked that buried carbon. Plant matter in ancient swamps became coal. Plankton on old sea floors became oil and gas.

So a lump of coal is a battery charged by the sun, long ago, and sealed shut for a geological age. When you burn it, you crack that seal and release the stored solar energy as heat — along with the carbon the plants pulled from the air, which we’ll come back to in Module 3. Every time you fill a tank or fire a furnace, you are spending sunlight that fell on a forest before the dinosaurs.

Why they won: energy packed tight

A fuel’s first great advantage is energy density — how much energy is packed into a given mass. Fossil fuels are extraordinary on this measure.

Petrol holds about 46 megajoules of energy per kilogram. (A megajoule is a million joules — recall from item 1 that a joule is the basic unit of energy.) A modern lithium-ion battery, the best rechargeable store we have, holds only about 0.7 megajoules per kilogram — roughly 0.2 kilowatt-hours per kilogram.

Do the division: 46 divided by 0.7 is about 65. Kilogram for kilogram, petrol carries roughly 65 times more energy than a battery. That is not a small edge. It is the difference between a fuel you can carry and a fuel that carries you down.

A worked example: the tank versus the battery

Put it in something physical. A 50-litre tank of petrol weighs about 37 kilograms when full. At 46 megajoules per kilogram, that tank holds about 1,700 megajoules of energy (46 × 37 ≈ 1,700).

Now store that same 1,700 megajoules in lithium-ion batteries. At 0.7 megajoules per kilogram, you need 1,700 divided by 0.7 — about 2,400 kilograms of battery. Nearly two and a half tonnes, to match one tank you can lift with one hand.

There is an honest caveat, and it comes straight from item 3. A petrol engine wastes most of its fuel as heat — only about a quarter of the energy turns the wheels. An electric motor is far better, using around 90 percent. So the usable gap is smaller than the raw one: factor in 25 percent versus 90 percent and the real-world advantage shrinks from about 65× to roughly 18×. But 18× is still enormous. Even after the electric motor’s efficiency claws back most of the difference, petrol wins on density by a wide margin. That margin is exactly why it became the natural fuel for anything that has to move.

Why they won: it sits and waits

Density is only half the story. The other half is storability — and here fossil fuels are almost magical.

Pour petrol into a sealed can and leave it. Months later, it is still petrol, still holding nearly all its energy, ready to burn the instant you want it. Coal can sit in a heap for years. Natural gas keeps in a tank. The energy doesn’t leak away. It waits, patient and portable, until you light it — and then it releases on demand, fast or slow, as much or as little as you ask.

Compare that to sunlight or wind. Sunlight only arrives when the sun is up. Wind blows when the weather decides, not when you need it. A battery slowly loses charge just sitting there, and a big one is heavy and costly. Fossil fuels solved the timing problem for free: dig up a store the sun made, and you hold concentrated, on-demand power in a form that keeps. That convenience is why they spread into every engine, furnace, and power station on Earth.

The whole, on the whole

It is tempting to think the world burns fossil fuels out of habit, or because powerful people want it that way. There is truth in both. But underneath sits a hard physical fact: these fuels are stunningly good at the one job a fuel has — to hold a great deal of energy in a small, light, patient package. That is not a trick of marketing. It is what a few hundred million years of buried sunlight produces.

Seeing that changes how you read the whole energy transition. The shift away from fossil fuels is not a fight against laziness or bad faith alone. It is a fight against real advantages baked into the physics — density and storability that nothing else yet matches cleanly. Whoever is working on it, wherever they sit, is up against the same stubborn numbers you just worked through. The reader filling a tank, the engineer building a battery, the planner weighing a grid — all of them are inside one system shaped by an inheritance the sun left buried long ago. Knowing why the old fuel is so good is the honest place to stand before asking what could ever replace it.