Energy, power and the units that measure them: Key terms, quiz and sources

Understanding Energy / Module 1: Energy, power and the units that measure them Wrap-up

Energy, power and the units that measure them: review

Key terms

Energy: The capacity to make things change, measured in joules, which appears as kinetic, potential, thermal, chemical, electromagnetic and nuclear energy.
Power: The rate at which energy is converted or delivered, measured in watts (joules per second).
Joule (J): The SI unit of energy, about the energy needed to lift a 100 gram apple by one metre.
Kilowatt-hour (kWh): The energy delivered by one kilowatt over one hour, equal to 3.6 megajoules; one 'unit' on an electricity bill.
Tonne of oil equivalent (toe): A unit of 41.868 gigajoules, roughly the energy in a tonne of crude oil, used in national energy balances.
Exajoule (EJ): 10^18 joules, the unit for world and large-country energy totals; the world used about 592 EJ in 2024.
Store and flow: Stores, such as coal and uranium, can be drawn on at will until depleted; flows, such as sunlight and wind, must be captured as they arrive.
Installed capacity: The maximum power a plant can produce, measured in watts, not the energy it actually delivers.
Capacity factor: A plant's actual annual output divided by what it would produce running at full power for all 8,760 hours.
Power density: The rate of energy flow per unit of area, in watts per square metre, used to compare land needs of sources and uses.
Gravimetric energy density: Energy stored per kilogram of a fuel or battery, in MJ per kg.
Volumetric energy density: Energy stored per litre or cubic metre, decisive for tanks, ships and pipelines.
Lower heating value: The heat released by burning a fuel when the water formed leaves as vapour, the usual basis for fuel energy contents.
Voltage and current: Voltage is the push driving electric charge; current is the rate of charge flow; their product is power.
Resistive (I2R) loss: Heat produced as current flows through a wire, rising with the square of the current, hence high-voltage transmission.
Transformer: A passive device of coils on an iron core that changes AC voltage with efficiencies above 99%.
Fission: The splitting of heavy nuclei such as uranium-235, releasing energy and neutrons that sustain a chain reaction.
Fusion: The joining of light nuclei such as hydrogen isotopes into helium, the process that powers the Sun.

Try it in the table

Open the Solar photovoltaics tile and find its typical capacity factor. Work out how many TWh a year 100 GW of panels would deliver in India, and compare that with the output of 100 GW of Nuclear fission reactors at a 90% capacity factor.
Open the Crude oil and Natural gas tiles and note their energy content per unit. Using the thousand-fold volume gap from this module, explain why the LNG chain tile lists liquefaction as its largest energy cost.
Open the Lithium iron phosphate batteries tile and find its energy density in Wh per kg. Convert it to MJ per kg and compare it with the 43 MJ per kg of kerosene in the Aviation propulsion tile.
Open the AC transmission tile and find the voltage levels it describes. Using the square law for resistive losses, estimate how much more a line would lose if it carried the same power at one third of that voltage.
Open the Uranium tile and the Thermal coal tile. Compare the tonnes of each needed to run a 1 GW plant for a year, and see which costs dominate in each case.
Open the Hydrogen and hydrogen carriers tile and compare hydrogen's energy per kilogram with its energy per litre at 700 bar. Note which carriers are proposed to get around the volume problem.

Final quiz

How many joules are in one kilowatt-hour?

One kilowatt is 1,000 J per second; an hour is 3,600 seconds; so 1 kWh is 3.6 million J, or 3.6 MJ.

Which statement about world energy in 2024 is correct?

The Statistical Review 2025 reports about 592 EJ, with oil, coal and gas together near 87%; non-hydro renewables were about 5.5%.

The average Indian's share of total energy supply is about 27 GJ a year. What continuous power is that?

27 x 10^9 J divided by about 31.6 million seconds is roughly 850 W.

A 1 GW nuclear plant runs at a 90% capacity factor. About how much electricity does it make in a year?

1 GW x 8,760 hours x 0.9 is about 7,900 GWh, or 7.9 TWh.

Which fuel has the highest energy per kilogram but one of the lowest energy per litre?

Hydrogen holds about 120 MJ per kg but even at 700 bar only about 5 MJ per litre, much less than diesel.

Why do long-distance transmission lines run at hundreds of kilovolts?

Power = voltage x current. Raising voltage cuts current, and resistive losses scale with current squared.

A press release says a new 300 MW solar park will 'generate 300 MW a year'. What is wrong?

Megawatts measure power. Annual energy is 300 MW x 8,760 h x 0.2, roughly 525,600 MWh or about 525 GWh.

Roughly what share of the world's electricity do electric motors use?

Motors in pumps, fans, compressors, conveyors and vehicles consume around half of all electricity, as Smil emphasises.

Next module: The laws of energy: conservation, entropy and efficiency →

Sources for this module

Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M1 'What energy is', 'Units and orders of magnitude', 'Energy versus power', 'Energy in matter: fuels and energy density', 'Electricity and the nucleus'
Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M1 'What energy is', 'Energy in matter: fuels and energy density'
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', 'Energy in matter: fuels and energy density'
Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M1 'Energy versus power'
Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M1 'Electricity and the nucleus'
Smil, Creating the Twentieth Century: Technical Innovations of 1867-1914 and Their Lasting Impact (2005) · via Smil Mastery Course, M1 'Electricity and the nucleus'
Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M1 'What energy is', 'Electricity and the nucleus'
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
Business Today, 'Coal plants' PLF to fall to 55% by FY32: Report' (reporting Ember analysis), 29 October 2025 · Indian coal plant load factor 69% in FY 2024-25. https://www.businesstoday.in/latest/economy/story/coal-plants-plf-to-fall-to-55-by-fy32-report-499990-2025-10-29
Outlook Business (PTI), 'Discoms post Rs 2,701 cr profit in FY25 after years of losses', citing Ministry of Power, January 2026 · AT&C losses 22.62% in FY 2013-14 and 15.04% in FY 2024-25. https://www.outlookbusiness.com/corporate/discoms-post-2701-cr-profit-in-fy25-after-years-of-losses
GE Vernova, 'Champa 800 kV HVDC: delivering efficient energy highways in India' (case study) · Champa-Kurukshetra +/-800 kV, 3,000 MW, 1,365 km; India's 765 kV substations. https://www.gevernova.com/electrification/sites/default/files/case_studies_pdfs/case-study-champa-800-kv-hvdc-delivering-efficient-energy-highways-in-india.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