Nobody wants energy for its own sake. People want a warm room, a cold drink, light after sunset, a ride to work and a phone that holds its charge. Energy is the means to those ends, and that plain point shapes every serious argument about how the world should power itself.

Energy services

Analysts call the things people actually want energy services: the heat, light, motion, cooling and communication that energy makes possible. The idea matters because the same service can be delivered with very different amounts of energy. An old incandescent bulb turns most of its electricity into heat, while an LED gives the same light from a small fraction of the power. So the question "how much energy does a society need?" has no fixed answer. It depends on what people want done and how efficiently they do it. This course is about how people get and use energy.

Primary and final energy

Experts count energy at different stages. Primary energy is energy in the form nature provides it: coal in the ground, crude oil, natural gas, sunlight, wind, falling water and uranium. Final energy is what reaches the user: gasoline at the pump, electricity at the socket, gas at the stove. Between the two lie losses. A coal power plant typically turns only about a third of the energy in its coal into electricity; the rest leaves as heat from cooling towers and chimneys.

This makes comparisons tricky. When a wind farm replaces a coal plant, it does not need to replace the two thirds of the coal's energy that was being thrown away as heat. Some statistics adjust for the heat that fuel-burning plants waste and some do not, so the same facts can make renewables look like a larger or smaller share. A careful reader asks which method a chart uses.

Where today's energy comes from

Fossil fuels still supply roughly four fifths of the world's primary energy. Oil is the largest single source, mostly burned in cars, trucks, ships and planes. Coal and natural gas follow, used for electricity, industry and heating. The rest comes from hydropower, nuclear power, wind, solar and biomass, which means wood, crop waste and dung.

Electricity is a smaller slice than many people assume. Electricity makes up only about a fifth of the energy people finally use; most energy is still burned directly as fuel for heat and transport. That is why cleaning up power stations, important as it is, is only part of the task.

Global energy use has also kept rising. Over the past two centuries it has grown more than twentyfold as population increased and people grew richer, and the world now burns more coal than at any earlier time. Historians of energy point out that past energy transitions were mostly additions, with new fuels piled on top of old ones rather than replacing them. The present transition asks for something without precedent: shrinking the old sources in absolute terms, within a few decades, while use keeps growing.

The energy ladder

How people meet their needs changes as incomes rise. Development researchers describe an energy ladder: as incomes rise, households move from wood and dung to charcoal and kerosene, and then to gas and electricity. Each rung is usually cleaner, more convenient and less time consuming. The picture is simplified. Many families "stack" fuels, using several at once, such as a gas stove for quick meals and a wood fire for slow ones.

Energy use per person varies enormously. The average person in the United States or Canada uses many times the energy of the average person in much of sub-Saharan Africa.

That gap is a question of human dignity as much as engineering. Clean light means children can study after dark. Refrigeration keeps vaccines and food safe. An honest account of the energy transition has to hold two goals at once: reducing the harms of fossil fuels, and giving billions of people more energy services, not fewer. Later levels return to both.

Worked example: one breakfast, two kitchens

Picture two breakfasts on the same morning. In an apartment in Lyon, France, a man boils water in an electric kettle, toasts bread and reads the news on his phone. His electricity comes mostly from nuclear reactors and hydropower dams many kilometers away, and he never sees a flame. In a village in rural Malawi a young woman rises before dawn, walks to gather firewood, and cooks porridge over an open fire inside her home.

Both kitchens deliver the same energy service: a hot breakfast. The Lyon kitchen relies on a long chain of plants and wires with losses along the way, but it is clean at the point of use. The Malawi kitchen sits on the bottom rung of the ladder. Its fuel costs no money but a great deal of time, and the smoke harms the lungs of everyone in the room, especially women and small children. Moving up the ladder would improve her life even before climate change enters the discussion. The lesson is to judge an energy system first by the services it provides and the harms it causes, and only then by its fuel.