How fast have energy transitions happened before?

Overlay a century of U.S. adoption curves, from electric service in homes to solar’s share of electricity, and see what sped some up and held others back

X3 of Energy Systems, Step by Step: read a century of U.S. adoption curves, from electricity in homes to solar’s share of electricity, and say how fast each change moved and why, from dated sources and with no forecast.
  • Foundations · L1
  • 12 min
  • After F10

U.S. adoption curves, as shares over time (each from a dated source)

Curve First point Last point Source
Homes with electric service 8.0% in 1907 98.8% in 1956 Census Bureau, Historical Statistics (1975), Series S 109-111
Town and other non-farm homes with electric service 47.4% in 1920 99.2% in 1956 Census Bureau, Historical Statistics (1975), Series S 109-111
Farm homes with electric service 1.6% in 1920 95.9% in 1956 Census Bureau, Historical Statistics (1975), Series S 109-111
Homes heated mainly with piped gas 11.3% in 1940 51.2% in 2000 Census Bureau, House Heating Fuel, 1940-2000
Homes heated mainly with coal or coke 54.7% in 1940 0.1% in 2000 Census Bureau, House Heating Fuel, 1940-2000
Families owning a car 54.0% in 1948 82.0% in 1970 Census Bureau, Historical Statistics (1975), Series Q 153 and Q 175
Solar's share of U.S. electricity 1.0% in 2015 8.7% in 2025 EIA, Electric Power Annual, Table 3.1.A
Every plotted point, with what each curve counts
Curve Points (year: share) What is counted Source
Homes with electric service 1907: 8.0%; 1912: 15.9%; 1917: 24.3%; 1920: 34.7%; 1925: 53.2%; 1930: 68.2%; 1935: 68.0%; 1940: 78.7%; 1945: 85.0%; 1950: 94.0%; 1955: 98.4%; 1956: 98.8% the share of all homes (dwelling units) with electric service Census Bureau, Historical Statistics (1975), Series S 109-111, Series S 109
Town and other non-farm homes with electric service 1920: 47.4%; 1925: 69.4%; 1930: 84.8%; 1935: 83.9%; 1940: 90.8%; 1945: 93.0%; 1950: 96.6%; 1955: 98.8%; 1956: 99.2% the share of town and other non-farm homes (urban and rural non-farm dwelling units) with electric service Census Bureau, Historical Statistics (1975), Series S 109-111, Series S 111
Farm homes with electric service 1920: 1.6%; 1925: 3.9%; 1930: 10.4%; 1935: 12.6%; 1940: 32.6%; 1945: 48.0%; 1950: 77.7%; 1955: 94.4%; 1956: 95.9% the share of farm homes (farm dwelling units) with electric service Census Bureau, Historical Statistics (1975), Series S 109-111, Series S 110
Homes heated mainly with piped gas 1940: 11.3%; 1950: 26.6%; 1960: 43.1%; 1970: 55.2%; 1980: 53.1%; 1990: 51.0%; 2000: 51.2% the share of occupied homes heated mainly with piped gas (the census's utility gas); the 1940 figure also counts bottled gas, so 1940 is not like for like Census Bureau, House Heating Fuel, 1940-2000, national rows, 1940 to 2000
Homes heated mainly with coal or coke 1940: 54.7%; 1950: 34.6%; 1960: 12.2%; 1970: 2.9%; 1980: 0.6%; 1990: 0.4%; 2000: 0.1% the share of occupied homes heated mainly with coal or coke (a fuel made by baking coal) Census Bureau, House Heating Fuel, 1940-2000, national rows, 1940 to 2000
Families owning a car 1948: 54.0%; 1950: 59.0%; 1955: 70.0%; 1960: 77.0%; 1965: 79.0%; 1970: 82.0% the share of families owning at least one car, from a household survey that began in 1947; Alaska and Hawaii are left out Census Bureau, Historical Statistics (1975), Series Q 153 and Q 175, Series Q 175
Solar's share of U.S. electricity 2015: 1.0%; 2017: 1.9%; 2019: 2.6%; 2021: 4.0%; 2023: 5.6%; 2025: 8.7% solar generation, utility-scale and small-scale (rooftop), as a share of all U.S. generation (our arithmetic on EIA's table) EIA, Electric Power Annual, Table 3.1.A, Table 3.1.A, 2015 to 2025
  • Town and other non-farm homes passed half between 1920 and 1925; farm homes between 1945 and 1950, about 25 years later.
  • Coal heated 54.7% of homes in 1940 and 2.9% in 1970; gas rose from 11.3% (with bottled gas) to 55.2%. Gas overtook coal as the most common main heating fuel between the 1950 and 1960 censuses.
  • Other fuels, such as heating oil, changed too; these census snapshots do not track homes switching from one fuel to another.
  • Families owning a car: 54% in 1948, 82% in 1970. Registered cars rose from 0.46 million in 1910 to 23.0 million in 1930.
  • Solar made 1.0% of U.S. electricity in 2015 and 8.7% in 2025, counting rooftop panels (our arithmetic on EIA's table).

With JavaScript on, you guess, switch the curves on one by one, zoom into solar's share and sort what sped each change up.

Real numbers, as published: each point comes from a dated U.S. source, the Census Bureau (Historical Statistics, 1975; housing censuses) or EIA. Solar's share is our arithmetic on EIA's table.

Need a refresher? F3 on the services people want from energy; F9 on who decides what in a city.

What you just did, as diagrams

Decades, not years
On its diffusion curve, electric service went from 8% of U.S. homes in 1907 to over 90% between 1945 and 1950.
Not everywhere at once
Farms passed half about 25 years after town homes; wires had to reach them first.
Gas overtook coal
Piped gas overtook coal as the most common main heating fuel between the 1950 and 1960 censuses. Other fuels changed too; the snapshots do not track homes switching.
What sped it
Networks built, public programmes and a cheaper or better service sped curves up; refining a new source until it could compete took decades.
Say what you measure
A device in homes, a fuel's share of a nation and a share of electricity move at different speeds.

U.S. energy transitions, dated

  1. 1850 · Coal at 9% of U.S. energy. Coal supplied 9% of U.S. energy use, and 27% by 1870 as railroads spread. Source: EIA, Today in Energy, 30 June 2026
  2. 1882 · Pearl Street Station. One of the world's first central power plants opened in New York; it could power 5,000 lights. Source: EIA, Energy Kids, electricity timeline
  3. 1885 · Coal passes wood. Around 1885 coal passed wood as the largest U.S. energy source. Source: EIA, Today in Energy, 30 June 2026
  4. 1907 · 8% of homes have electric service. The first year of the Census Bureau's series of homes with electric service. Source: Census Bureau, Historical Statistics (1975), Series S 109-111
  5. 1908 · Model T. Ford first produced the Model T car. Source: EIA, Energy Kids, transportation timeline
  6. 1936 · Rural Electrification Act. A law aimed at bringing electricity to farms across the country. Source: EIA, Energy Kids, electricity timeline
  7. 1950 · Petroleum passes coal. Petroleum became the most-used U.S. energy source, and has been ever since. Source: EIA, Today in Energy, 30 June 2026
  8. 1956 · Interstate Highway System. The Federal-Aid Highway Act of 1956 established the Interstate Highway System. Source: EIA, Energy Kids, transportation timeline
  9. 1958 · Natural gas passes coal. Natural gas passed coal in U.S. energy use; it has been second most years since. Source: EIA, Today in Energy, 2 July 2025
  10. 1970 · Gas heats most homes. Piped gas heated 55.2% of homes and coal or coke 2.9%, against 11.3% (with bottled gas) and 54.7% in 1940. Source: Census Bureau, House Heating Fuel, 1940-2000
  11. 2023 · Renewables pass coal. U.S. energy use from renewables passed coal for the first time since the 1880s. Source: EIA, Today in Energy, 30 June 2026
  12. 2025 · Solar at 8.7% of electricity. Solar made 8.7% of U.S. electricity, utility-scale and small-scale together (our arithmetic). Source: EIA, Electric Power Annual, Table 3.1.A

Where we stop

This side topic stops at reading the curves

Adoption enters the course only as diffusion, a share over time; costs and forecasts stay out.

This side topic covers

  • What share of homes, families or electricity each change reached, and when.
  • What sped some curves up and held others back, read from dated sources.
  • Why the answer to 'how fast' depends on what you measure.

Left to specialists

  • How the cost of any technology changed over time.
  • Forecasts of how fast anything spreads next.
  • Economic history beyond these curves: prices, wages, trade.

Read next, three rungs

  1. Start here. Long-run charts of the world's energy sources, with how each series is built. Our World in Data, Energy: Energy Sources and Transitions (Ritchie, Rosado and Roser), published 20 December 2023, read 7 November 2026
  2. Go further. How long national and global transitions took, country by country. Vaclav Smil, Energy Transitions: Global and National Perspectives (Praeger), cited by name, second edition, 2017
  3. Go deep. Energy across human history, from muscle power to fossil fuels and electricity. Vaclav Smil, Energy and Civilization: A History (MIT Press), cited by name, 2017
Go deeper · AnalystReading a diffusion curve without fooling yourself
  • Say the whole. A share of homes, of families and of electricity are three different wholes; a curve means nothing until its denominator is named.
  • Published points only. The page reports a year as “between 1945 and 1950” rather than drawing a smooth line through gaps.
  • Starts and ends. The car survey began when most families already had a car; solar’s curve stops at the latest year of data.
  • Services first. Grubler (2012, abstract) points to energy end use and services, and to the lengthy process of transitions.
Go deeperWhat this lesson leaves out
  • Countries other than the United States, and the world's energy mix before 1900.
  • Prices, wages and trade: the economic history behind each curve.
  • How the cost of any technology changed over time.
  • Any forecast of how fast a curve will move next.

Check yourself

About a minute: two myths.

Myth or fact? “Farm homes and town homes passed half with electric service at about the same time.”

  • Myth
  • Fact
Answer

Myth. Town and other non-farm homes passed half between 1920 and 1925; farm homes only between 1945 and 1950, about 25 years later (Census Bureau).

Myth or fact? “Petroleum has been the largest source of U.S. energy since 1950.”

  • Myth
  • Fact
Answer

Fact. Petroleum passed coal in 1950; coal had passed wood around 1885 (EIA, 2026).

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
about 40 years From 8% of U.S. homes with electric service in 1907 to 90%, passed between 1945 (85.0%) and 1950 (94.0%). Census Bureau, Historical Statistics (1975), Series S 109-111 · 1975
about 25 years How far farms trailed town homes: town and other non-farm homes passed half between 1920 and 1925, farm homes between 1945 and 1950. Census Bureau, Historical Statistics (1975), Series S 109-111 · 1975
54.7% → 2.9% Homes heated mainly with coal or coke, 1940 to 1970, while piped gas rose from 11.3% (1940, with bottled gas) to 55.2%. Census Bureau, House Heating Fuel, 1940-2000 · decennial censuses 1940 to 2000
1885 · 1950 Coal passed wood as the largest U.S. energy source around 1885; petroleum passed coal in 1950. EIA, Today in Energy, 30 June 2026 · 30 June 2026
8.7% Solar's share of U.S. electricity in 2025, utility-scale and small-scale together (our arithmetic on EIA's table). EIA, Electric Power Annual, Table 3.1.A · 2025 data

Keep going

Back to the course map

Sources and further reading

Spotted an error, or stuck? Write to hlli@ksu.edu. Corrections are made in place and dated here.

Text licensed CC BY 4.0, code MIT: teachers and students may reuse and adapt it, with credit to Honglin Li.