The history
260 years from a glass box in the Alps to tens of millions of rooftop tanks, from Miami to Bengaluru.
People have always known that dark things get hot in the sun. The trick was keeping that heat: first under glass, then in an insulated tank, then inside a vacuum. Again and again solar hot water boomed when fuel was dear and faded when it was cheap, until laws in Israel, Barcelona and Bengaluru, and cheap Chinese tubes, put it on roofs for good.
1767
Glass-covered solar hot box
Horace-Bénédict de Saussure, Switzerland
1891
Solar water heater sold commercially
Clarence Kemp's Climax, USA
1909
Separate collector and insulated tank (thermosiphon)
William J. Bailey's Day and Night, USA
1911
Evacuated-tube collector patent
William L. R. Emmet, USA
1955
Selective absorber coatings shown
Harry Zvi Tabor, Israel
1980
Nationwide solar water heater law (widely considered the first)
Israel
1999–2000
European city solar ordinance
Barcelona, Spain
2009
Indian city tying solar heaters to power connections
BESCOM, Bengaluru
1760 – 1890
Scientists learn that glass traps the sun's heat, and people warm water in black tanks that go cold every night.
1767
A Swiss scientist built a wooden box with three layers of glass on top and a blackened inside. Left in the sun, it reached about 110 °C, hot enough to cook fruit. He carried it up and down a mountain and found the outside air made little difference.
Why it mattered. It was the first careful experiment with the greenhouse trick every solar collector uses: glass lets sunlight in and keeps heat from getting out.
1835
1830s
While mapping the southern stars, the British astronomer used a glass-covered box, set in the sun, to cook food. It is one of the earliest recorded uses of a solar hot box on an expedition.
Why it mattered. It showed the hot box was useful, not just a curiosity.
1878
A French teacher built big mirrored cones that focused sunlight on a boiler. At the 1878 Paris World's Fair his solar machine even ran an ice maker, and won a gold medal. Cheap coal soon made it look unnecessary.
Why it mattered. It proved sunlight could heat water to boiling, and showed how fuel prices decide whether solar wins.
1885
Before 1900
Before any company sold them, people in sunny parts of the US painted metal water tanks black and set them on roofs or in the yard. The water got warm by afternoon but went cold again overnight.
Why it mattered. It was the simplest solar water heater, and its big weakness, losing heat at night, set the next problem to solve.
1891 – 1950
The first heaters for sale, the thermosiphon tank and the vacuum-tube idea, until cheap gas and electricity end the boom.
1891
28 April 1891
Kemp patented a heater made of black metal tanks inside a glass-topped, insulated wooden box, fitted on the roof. He called it the Climax. It was the first solar water heater sold commercially.
Why it mattered. Putting the tank in an insulated glass box kept it warmer for longer, the basic idea behind every batch heater since.
1909
Bailey split the heater in two: a thin collector of copper pipes under glass, and a separate insulated tank placed higher up. Hot water rose into the tank on its own and stayed hot until the morning, which gave the heater its name.
Why it mattered. This is the design of almost every thermosiphon system on Indian roofs today.
1911
An American engineer patented solar collectors that put the absorber inside an evacuated glass tube, like a vacuum flask. The glass-making needed to build them cheaply did not exist yet, so the idea waited for decades.
Why it mattered. It is the first known design of the evacuated tube that now dominates the world market.
1913
1912–1913
Shuman built long curved mirrors that focused sunlight on pipes of water, making steam to drive a pump for irrigating cotton fields. The First World War and cheap oil ended the project.
Why it mattered. It showed sunlight could heat water on an industrial scale.
1930
1920s–1930s
Big finds of natural gas near Los Angeles made gas water heating cheap and easy. Sales of solar heaters in California collapsed, and Bailey sold his patent rights to a company in Florida.
Why it mattered. Solar water heating has always lived or died by the price of the fuel it replaces.
1941
By 1941
In fast-growing Florida, where electricity was costly, the Day and Night design took off. By 1941 more than 60,000 had been installed, and over half the people of Miami heated their water with the sun. After the war, cheap electricity ended the boom.
Why it mattered. It was the biggest solar water heating market in the world at the time.
1950 – 1985
Countries short of fuel turn to the sun, a physicist invents the selective coating, and Israel writes it into law.
1953
Short of fuel after independence, Israel looked to the sun. Engineer Levi Yissar built a prototype heater and in 1953 started NerYah, the country's first commercial maker.
Why it mattered. It began the most successful national solar water heating story of the century.
1955
Tabor, a physicist, found how to make black metal surfaces that soak up sunlight but give off little infrared heat. He showed his collectors at the first world solar energy symposium in Arizona in 1955.
Why it mattered. Selective coatings roughly doubled collector performance at useful temperatures, and every good collector now has one.
1967
Cheap, reliable flat-plate heaters spread across Israel's rooftops. By 1967 about one household in five used one.
Why it mattered. It proved that ordinary families would choose solar hot water when it was good value.
1968
Late 1960s
The first solar water heater in Cyprus went up in Nicosia in the late 1960s, based on Israeli designs. Local workshops built them, and they spread even to villages not yet on the electricity grid. Today over 90% of Cypriot homes use one.
Why it mattered. A small island showed that almost every home can get its hot water from the sun.
1973
When oil prices shot up in 1973, interest in solar water heating returned around the world, with research money and tax credits. When oil got cheap again in the 1980s, many markets shrank once more.
Why it mattered. It started the modern research that led to today's tubes and coatings.
1980
Israel passed a law requiring solar water heaters on all new homes, except tall towers. It was the first country-wide rule of its kind. Today about 85% of Israeli households get hot water from the sun.
Why it mattered. It showed that building rules could make solar hot water normal, an idea many Indian cities later copied.
1975 – today
Evacuated tubes become cheap enough to make by the million, and cities from Barcelona to Rizhao require solar hot water.
1975
1975–1980
Researchers at the University of Sydney worked on an all-glass evacuated tube: two glass tubes sealed together with a vacuum between, and a selective coating sputtered onto the inner one. By about 1980 tubes based on their work were being made in Japan.
Why it mattered. It turned Emmet's old idea into a tube that could be made in a factory.
1985
Yin developed a graded aluminium-nitride coating that could be sputtered onto glass tubes cheaply. From 1993 he helped build factories that made millions of tubes a year.
Why it mattered. It made evacuated tubes cheap enough to sell by the million, which is why China, and later India, filled their roofs with them.
2000
1999–2000
Barcelona became the first European city to require solar water heating in new and renovated buildings, from August 2000, supplying 60% of their hot water. The collector area in the city grew more than 50 times in ten years.
Why it mattered. It became a model for building rules across Spain and Europe.
2001
From 2001
A Chinese coastal city combined building rules and public education to put solar water heaters on nearly every home: 99% of households in its central districts use one.
Why it mattered. It showed what a determined city can do with cheap, locally made tubes.
2023
End of 2023
Solar thermal collectors around the world added up to about 560 GW of heating capacity, with China far ahead of every other country. About 122 million solar water heating systems were in use by the end of 2022.
Why it mattered. Solar water heating quietly supplies more energy than many people realise.
By the numbers
Global solar thermal capacity in operation, as compiled by the IEA Solar Heating and Cooling Programme.
1982 – today
Government programmes, Indian standards and city by-laws put millions of square metres of collectors on Indian roofs.
1982
India set up a department for renewable energy, which later became the Ministry of New and Renewable Energy (MNRE). Solar water heating was one of its first programmes, after pilot projects going back to the 1960s.
Why it mattered. It began four decades of Indian support for solar water heaters.
1990
c. 1990 (revised 2003)
The BIS published IS 12933 for solar flat-plate collectors, setting rules for materials, tests and performance. It was revised in 2003 into several parts, and makers must carry the ISI mark.
Why it mattered. A standard lets buyers trust that a collector will actually perform and last.
2007
2007 (enforced from 2009)
Karnataka required solar water heaters on new homes of 600 square feet and more. From April 2009 the power company BESCOM refused regular electricity connections to homes without one. Pune and other Maharashtra cities have similar rules.
Why it mattered. Tying the rule to the electricity connection made it stick, and Bengaluru's roofs filled with tubes.
2021
End of 2021
About 18.2 million square metres of solar collectors were working in India, close to the 2022 target. Around 80% of it was evacuated tubes.
Why it mattered. India had become one of the world's big solar hot water markets.
2024
October 2024
MNRE issued a Quality Control Order making BIS certification compulsory for solar flat-plate collectors and evacuated-tube systems sold in India, under IS 12933, IS 16542 and IS 16544.
Why it mattered. It aims to keep poor-quality heaters off Indian roofs.
By the numbers
Collector area in operation in India. It grew about 150 times in 32 years; around 80% is now evacuated tubes.
Did you know?
By 1941 more than half the people of Miami heated their water with the sun.
About 85% of households in Israel and over 90% in Cyprus get their hot water from the sun.
Around 80% of India's solar water heater collectors are evacuated glass tubes.
A cracked evacuated tube gives itself away: the shiny silver patch at its bottom turns milky white.
In Bengaluru, from 2009 a new home without a solar water heater could not get a regular electricity connection.
The people
1740 – 1799 · Physicist and alpinist · Switzerland
Built the glass-topped hot box that first showed how glass traps the sun's heat.
1792 – 1871 · Astronomer · England
Cooked with a solar hot box at the Cape of Good Hope in the 1830s.
1825 – 1912 · Teacher and inventor · France
Built solar boilers and a sun-powered ice maker shown in Paris in 1878.
active 1890s · Inventor and manufacturer · USA
Patented the Climax, the first solar water heater sold commercially.
active 1900s–1920s · Engineer · USA
Invented the Day and Night heater, with its separate collector and insulated tank.
1859 – 1941 · Electrical engineer · USA
Patented an evacuated-tube solar collector in 1911.
1862 – 1918 · Inventor · USA
Built a solar steam plant in Egypt in 1913.
20th century · Engineer · Israel
Built Israel's first solar heater prototype and founded NerYah in 1953.
1917 – 2015 · Physicist · Israel
Invented selective black coatings, often called the father of Israeli solar energy.
20th–21st century · Engineer and professor · China
Developed the low-cost sputtered coating behind China's mass-produced evacuated tubes.
Where it happened
Sources
Dates marked “c.” are approximate, and historians sometimes disagree about who was first. If you spot a mistake, tell us.
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