Energy is not the same thing as electricity
People often use “energy” and “electricity” as if they were one thing. Electricity is a convenient carrier of energy: it can be made from many different sources, sent over long distances and turned into light, motion or heat at the other end. But a large share of the energy people use never becomes electricity at all. Gasoline in a car, natural gas in a furnace and diesel in a truck are all used directly as fuel.
Under the idea of energy conservation, energy is never created or destroyed, only converted from one form to another. Every source below is really a way of capturing energy that was already stored in chemical bonds, in atomic nuclei, in sunlight or in moving air and water, and converting it into a form we can use.
The main families of energy sources
Fossil fuels
Coal, oil and natural gas formed over very long periods, typically millions of years, from the remains of ancient organisms buried and compressed underground. Much of the energy in them traces back to sunlight captured by plant photosynthesis long ago. Burning them releases that stored chemical energy as heat.
Fossil fuels powered the Industrial Revolution and are still a major part of the world’s energy supply. They are compact, easy to store and easy to transport. The trade-offs are that they are finite on human timescales, and burning them releases carbon dioxide, which strengthens the greenhouse effect. Burning them also produces air pollutants, depending on the fuel and the equipment.
Nuclear power
Nuclear power plants get heat from nuclear fission, in which the nuclei of heavy atoms such as uranium split and release energy. That heat is used in much the same way as heat from burning coal: to boil water into steam. A plant produces large amounts of electricity from a small amount of fuel and does not burn anything, so it does not release carbon dioxide while operating. It also produces radioactive waste that must be managed carefully, and plants are expensive and slow to build. How much nuclear power a country should use is a matter of ongoing public debate.
Renewable sources
Renewable sources are ones that are naturally replenished on human timescales: sunlight, wind, flowing water, heat from within the Earth, and plant material. Solar panels convert light directly into electricity, while wind turbines and hydroelectric dams spin generators. Each has its own strengths and limits, and they are compared in more detail in our article on renewable energy.
From heat to electricity
Most large power plants, whether they burn coal or gas or use nuclear fission, share a basic design. Heat boils water, the steam spins a turbine, and the turbine turns a generator. Some gas plants skip the steam step and spin a turbine with hot combustion gases directly.
A generator works by electromagnetic induction, discovered by Michael Faraday in the 1830s. When a coil of wire moves through magnetic fields, an electric current is pushed through the wire. Wind and hydro turbines use the same principle. Only solar panels, which have no moving parts, make electricity in a fundamentally different way.
Because no conversion is perfect, some energy is always lost as waste heat at each step. This is why a thermal power plant typically delivers only a fraction of the fuel’s energy as electricity.
How electricity reaches a home
Power from a plant takes a fairly standard journey.
- Step up. Transformers near the plant raise the voltage to very high levels.
- Transmission. High-voltage lines carry the power across long distances. Higher voltage means less current is needed for the same power, and less current means less energy wasted as heat in the wires.
- Substations. Closer to cities, transformers lower the voltage in stages.
- Distribution. Local lines carry the power along streets, and a final transformer reduces it to the voltage used in homes.
Transformers work only with alternating current, which is one major reason power grids use it. Alternating current simply reverses direction many times a second.
Keeping supply and demand matched
Electricity is difficult to store in large quantities, so a grid must produce nearly as much as people are using at every moment. Operators adjust output through the day, bringing plants up or down as demand rises in the evening and falls overnight. Wind and solar output depends on the weather and the time of day, which makes this balancing job more complicated and is one reason battery storage and other ways of storing energy get so much attention.
What this means in everyday life
Every device you plug in is at the end of a long chain of conversions. The exact mix of sources behind your electricity depends heavily on where you live, and it changes over time as plants are built and retired. Figures on how much each source contributes vary by country and by year, so it is best to check a recent national or regional report rather than rely on a single global number.