Muscles, wood, water and wind: energy before fossil fuels: Key terms, quiz and sources

Understanding Energy / Module 4: Muscles, wood, water and wind: energy before fossil fuels Wrap-up

Muscles, wood, water and wind: energy before fossil fuels: review

Key terms

Basal metabolic rate (BMR): The energy a body needs at rest to stay alive, about 7 MJ a day or 80 W for a 70 kg adult.
Physical activity level (PAL): The ratio of total daily energy use to basal metabolism, from about 1.4 for desk work to over 2.0 for heavy labour.
Muscle efficiency: The share of food energy burned by muscles that becomes mechanical work, about 20-25%.
Persistence hunting: Running an animal to exhaustion in the heat, made possible by human sweating.
Foraging: Living by gathering wild plants, hunting and fishing, governed by the energy return on effort.
Shifting cultivation: Clearing and burning a plot, cropping it for a few years and leaving it to regrow for a decade or more.
Land productivity: Output of food per hectare; farming's main advantage over foraging.
Crop rotation: Alternating crops, usually including legumes and fodder, to restore soil nitrogen and feed animals.
Organic ceiling: The limit on yields, population and output set by dependence on current photosynthesis and recycled nutrients.
Prime mover: Any organism or machine that converts a primary energy flow into mechanical work.
Collar harness: A padded collar resting on a horse's shoulders, letting it pull with full strength without choking.
Horsepower: James Watt's unit of power, 33,000 foot-pounds per minute or about 745.7 W, close to a good working horse's sustained output.
Overshot waterwheel: A wheel turned by the weight of water poured into buckets at its top, the most efficient traditional design.
Betz limit: The maximum share of the wind's kinetic energy a rotor can capture, 16/27 or about 59%.
Charcoal: Nearly pure carbon made by heating wood with little air, with about 29-30 MJ/kg.
Bloomery: An early iron furnace that produced a spongy mass of iron and slag instead of molten iron.
Clean cooking access: Use of cooking fuels and stoves, such as LPG, electricity, biogas or ethanol, that do not cause harmful indoor smoke.
Fuel stacking: Household use of several cooking fuels at once, such as LPG alongside firewood.

Try it in the table

Open the Muscle power tile and compare its stated sustained output with the 50-100 W figure here, then look at Rockets and human power for where human power is still used today.
Open Energy in farming and find what replaced bullocks and horses: list the diesel, electricity and fertiliser inputs and compare their power with an ox at 250-400 W.
Compare Small, airborne and traditional wind with Onshore wind. Note the rotor sizes and outputs, and check how close each comes to the Betz limit of 59%.
Go from Water cycle to Run-of-river and small hydro and estimate the power of a stream using flow x height x 9.8 kW, as in the overshot mill example.
Open Firewood and charcoal supply and find the charcoal yield from wood. Then open Boilers, furnaces and stoves and compare the efficiency of an open fire with an LPG or improved biomass stove.
Look at Biomass and at CNG, LPG and biogas distribution, and identify which clean cooking options are most relevant for India's remaining 430 million people without clean cooking.

Final quiz

Roughly what share of a sedentary adult's daily energy goes to basal metabolism?

Basal metabolism dominates for people not doing heavy physical work.

According to Smil's estimate, about how many workers were needed at any one time to build the Great Pyramid of Khufu?

Energy calculations suggest under 10,000, probably under 7,000, over about two decades; the hard part was organisation.

Typical medieval European wheat yields were about:

Often below 1 t/ha with three to five grains harvested per grain sown.

What finally broke the nitrogen limit on traditional farming?

Ammonia synthesis supplied nitrogen far beyond what manure and legumes could recycle.

The Domesday Book of 1086 recorded about how many water mills in England?

5,624 mills, roughly one for every 350 people.

Which preindustrial machine commanded the most power?

Hundreds or thousands of square metres of canvas captured far more wind power than any land-based mill.

A traditional kiln turns 20% of a tonne of wood (16 MJ/kg) into charcoal (30 MJ/kg). What share of the wood's energy ends up in the charcoal?

200 kg x 30 MJ = 6,000 MJ out of 16,000 MJ, about 38%; more than half the energy is lost.

Why is the number of people without clean cooking rising in sub-Saharan Africa while falling in Asia?

IEA and Tracking SDG7 data show Asian access rising fast with LPG, while African progress has not kept up with population growth.

Next module: The fossil-fuel revolution →

Sources for this module

Smil, Energy and Civilization: A History (2017) · via Smil Mastery Course, M3 'Limits of human performance', 'Foraging: gathering, hunting, fishing', 'Why agriculture began', 'Traditional farming and its intensification', 'The limits of traditional farming'; M4 all units
Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M3 'The human body as an energy converter', 'Limits of human performance', 'Foraging: gathering, hunting, fishing', 'Traditional farming and its intensification'; M4 'Prime movers: muscles and animals', 'Wind: mills and sails'
Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M3 'The human body as an energy converter'
Smil, Should We Eat Meat? Evolution and Consequences of Modern Carnivory (2013) · via Smil Mastery Course, M3 'The human body as an energy converter', 'Foraging: gathering, hunting, fishing'
Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M3 'Limits of human performance'; M4 'Moving and building before engines'
Smil, Grand Transitions: How the Modern World Was Made (2021) · via Smil Mastery Course, M3 'Why agriculture began', 'The limits of traditional farming'
Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M3 'Why agriculture began'
Smil, Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production (2001) · via Smil Mastery Course, M3 'Traditional farming and its intensification', 'The limits of traditional farming'
Smil, China's Past, China's Future: Energy, Food, Environment (2004) · via Smil Mastery Course, M3 'The limits of traditional farming'
Smil, Energy in World History (1994) · via Smil Mastery Course, M4 'Prime movers: muscles and animals', 'Water wheels', 'Wind: mills and sails', 'Moving and building before engines', 'Metals and warfare in the organic age'
Smil, Growth: From Microorganisms to Megacities (2019) · via Smil Mastery Course, M4 'Water wheels'
Smil, Biomass Energies: Resources, Links, Constraints (1983) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'
Smil, Energy in the Developing World: The Real Energy Crisis (ed. with W. Knowland, 1980) · via Smil Mastery Course, M4 'Wood, charcoal and other biomass fuels'
Smil, Still the Iron Age: Iron and Steel in the Modern World (2016) · via Smil Mastery Course, M4 'Metals and warfare in the organic age'
Smil, Two Prime Movers of Globalization: The History and Impact of Diesel Engines and Gas Turbines (2010) · via Smil Mastery Course, M4 'Moving and building before engines'
World Health Organization, 'Household air pollution' fact sheet (16 December 2025) · about 2.1 billion people cook with open fires or inefficient stoves using solid fuels or kerosene; about 2.9 million deaths in 2021, including over 309,000 children under five; https://www.who.int/en/news-room/fact-sheets/detail/household-air-pollution-and-health
IEA, IRENA, UN Statistics Division, World Bank and WHO, Tracking SDG7: The Energy Progress Report 2025 (data analysis, SEforALL) web ↗ · 2,087 million people without clean cooking in 2023 (Asia 983 million, Africa 975 million), down from 2,999 million in 2010; 1.7-1.8 billion projected for 2030; https://www.seforall.org/system/files/2025-12/SEforALL%20SDG7_TRACKING_2025-data_analysis.pdf
IEA, SDG7: Data and Projections, 'Access to clean cooking' (2024) · India about 430 million without access in 2023, projected 285 million by 2030; China 146 million; progress in developing Asia driven largely by LPG programmes; https://www.iea.org/reports/sdg7-data-and-projections/access-to-clean-cooking
IEA, Clean Cooking in Africa 2026, Executive summary (2026) · nearly two billion people still cook with wood, charcoal and other polluting fuels; about 2.5 million premature deaths a year; about four hours a day spent collecting fuel and cooking; 1.2 Gt CO2-eq a year from traditional cooking; https://www.iea.org/reports/clean-cooking-in-africa-2026/executive-summary
Energy Institute, Statistical Review of World Energy 2025 web ↗ · 2024 world primary energy 592 EJ, covering commercially traded fuels; https://www.energyinst.org/statistical-review
Wrigley, Energy and the English Industrial Revolution, Cambridge University Press (2010) · about 15 million acres of woodland needed to match the energy Britain got from coal in 1800; review in Times Higher Education, https://www.timeshighereducation.com/books/britons-who-saw-coal-fuel-nations-rise-and-rise/193623.article
Government of India, Cabinet decision on PMUY targeted subsidy for 2025-26 (PM India / PIB, 8 August 2025) · PMUY launched May 2016; about 10.33 crore connections as of 1 July 2025; Rs 300 subsidy per 14.2 kg cylinder for up to 9 refills a year; average refills per PMUY household about 3 (2019-20), 3.68 (2022-23), about 4.47 (2024-25); https://www.pmindia.gov.in/en/news_updates/cabinet-approves-continuation-of-targeted-subsidy-for-pradhan-mantri-ujjwala-yojana-consumers-for-2025-26-at-rs-12000-crore/