Energy, power and the units that measure them: Units and orders of magnitude

2.1 The joule is small, so energy accounting uses bigger units

The scientific unit of energy is the joule (J). It is a small amount. Lifting a 100 gram apple by one metre takes about one joule. A single digestive biscuit holds around 300,000 joules. So in practice people use larger units, and different industries grew up with different ones.

Nutritionists use the calorie. One calorie is 4.184 joules, and the Calorie on a food label is really a kilocalorie, 4,184 joules. A daily diet of 2,000 kilocalories is about 8.4 million joules, or 8.4 megajoules (MJ).

The electricity industry uses the kilowatt-hour (kWh): one kilowatt running for one hour. Because an hour has 3,600 seconds and a kilowatt is 1,000 joules per second, one kWh is 3.6 MJ. That 2,000 kilocalorie diet is about 2.3 kWh, less than a household electric geyser uses in a long morning.

The oil industry counts in barrels. A barrel is 159 litres, and a barrel of crude holds about 6.1 gigajoules (GJ). International energy statistics often use the tonne of oil equivalent (toe), defined as 41.868 GJ, roughly the energy in a tonne of crude oil. Coal is sometimes counted in tonnes of coal equivalent, about 29.3 GJ, because a tonne of typical coal holds less energy than a tonne of oil. India's coal statistics are in physical tonnes, and Indian thermal coal is often of fairly low energy content, so a tonne of it can hold well under 20 GJ.

The British thermal unit (BTU), about 1,055 J, survives in American gas markets, where prices are quoted per million BTU. A million BTU is just over one gigajoule, a handy approximation when comparing LNG prices with anything else.

All of these are the same quantity in different clothes.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Units and orders of magnitude'
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M1 'Units and orders of magnitude'

2.2 Prefixes climb in steps of a thousand, and mixing them up is the costliest mistake

Energy quantities are scaled with prefixes, and each step multiplies by a thousand. Kilo is a thousand (10^3), mega a million (10^6), giga a billion (10^9), tera a trillion (10^12), peta 10^15 and exa 10^18. A household in Delhi might use a few thousand kilowatt-hours of electricity a year. A large power station produces several terawatt-hours. India's power system generates on the order of two thousand terawatt-hours a year. The whole world uses several hundred exajoules of all forms of energy.

Smil argues that most public misunderstanding of energy comes not from errors in the fourth significant figure but from confusing whole orders of magnitude: thousands with millions, millions with billions. A factor-of-a-thousand error is worse than any rounding. One way to feel the difference is through time. A million seconds is about 11.6 days. A billion seconds is about 31.7 years. Mistaking one for the other is the difference between a fortnight and most of a career.

Two habits help. First, normalise. Totals tell you little until they are divided by something: energy per person per year, megajoules per kilogram, watts per square metre. Normalised numbers allow fair comparisons between a country of 1.45 billion people and one of 50 million. Second, be honest about precision. Most energy and environmental data carry uncertainties of 10% or more. Smil deliberately rounds and gives ranges. Quoting a national energy total to six digits signals false confidence, not knowledge.

A practical test before trusting any energy claim is to estimate it yourself to the nearest power of ten. If a press release says a project will power a million homes, multiply a million by a typical home's annual use and compare that with the plant's likely output. The answer is usually clear within a minute.

• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M1 'Units and orders of magnitude'
• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'Units and orders of magnitude'

2.3 The world used 592 exajoules in 2024, and the gap between countries is tenfold

According to the Energy Institute's Statistical Review of World Energy 2025, the world's total energy supply reached a record of about 592 EJ in 2024, roughly 2% more than in 2023. That is about 14 billion tonnes of oil equivalent, or about 164,000 TWh. Oil supplied about 34%, coal 28%, natural gas 25%, nuclear 5%, hydropower about 3% and other renewables about 5.5%. Fossil fuels together still met about 87% of the total.

That 2025 edition changed how it counts. It now uses the physical energy content method, which records wind, solar and hydro by the electricity they produce rather than by the fossil fuel they displace. Older editions, which inflated those sources by about two and a half times, gave a higher world total, around 620 EJ for 2023. The physical world did not change; the accounting did. The next module explains both methods.

Totals become meaningful when divided by population. With about 8.2 billion people, 592 EJ works out at roughly 73 GJ per person per year. The Review puts the United States at about 266 GJ per person. Dividing its country totals by 2024 populations gives (calculated) the European Union about 116 GJ, China about 112 GJ and India about 27 GJ, so the average American uses roughly ten times as much energy as the average Indian.

Another way to read those numbers is as a continuous power. A year has about 31.6 million seconds, so 73 GJ per year is an average flow of about 2,300 watts per person, around 23 times the 100 W a human body runs on. The average Indian draws about 850 W, the average Chinese or European about 3,600 W, the average American about 8,400 W. These figures include energy used in industry, transport and power stations on people's behalf, not only what passes through their homes.

• Energy Institute, Statistical Review of World Energy 2025 (74th edition), Energy Institute with KPMG and Kearney, 2025 web ↗ · Key highlights p. 4; total energy supply by country p. 13; by fuel p. 14; per capita p. 15; methodology p. 8. https://www.energyinst.org/statistical-review (copy at https://assets.kpmg.com/content/dam/kpmg/tr/pdf/2025/07/Statistical_Review_of_World_Energy.pdf)
• Energy Institute, Statistical Review of World Energy 2025: Methodology, 2025 web ↗ · Physical energy content method and efficiency factors. https://www.energyinst.org/__data/assets/pdf_file/0003/1658154/Methodology-2025.pdf
• Our World in Data, energy dataset (owid-energy-data.csv), 2026 · 2024 population estimates (UN World Population Prospects) used to calculate per-person values. https://github.com/owid/energy-data

See it in the table

Check your understanding

How many kilowatt-hours are in one gigajoule?

World energy supply was about 592 EJ in 2024 for about 8.2 billion people. What is that per person?

Why did the 2025 Statistical Review report a lower world total than older editions would have?