50 Fun Facts About Solar Energy

By a 2002 estimate, the sunlight Earth absorbs in what span held more energy than the world used all year?

About one hour of sunshine carried more energy than humanity burned through in twelve months. By 2019, with demand far higher, the span had stretched to under an hour and a half.

Which US president put solar water heaters on the White House roof, only for his successor to remove them?

Jimmy Carter's panels heated water for the building until they came down under Ronald Reagan. Barack Obama had new panels fitted to the roof in 2013, the first to power the president's living quarters.

On which kind of day does a typical solar panel convert sunlight into electricity most efficiently?

Cold and sunny days suit panels best, because a solar cell loses roughly 0.45% of its efficiency for every degree Celsius it warms. Cells in full sun often run 20 degrees hotter than the air.

Which ancient Greek is said to have set Roman ships ablaze at Syracuse by focusing sunlight with mirrors?

Archimedes gets the credit only in later retellings: the three earliest accounts of the siege never mention mirrors. Whether the trick could work has been argued over since the Renaissance.

Which NASA Mars rover fell silent in 2018 after a planet-wide dust storm starved its solar panels?

Opportunity was built to last 90 Martian days and kept working for more than 14 years. Engineers hoped a windy season would sweep its panels clean, but it never called home again.

Roughly how long does sunlight take to cross the 150 million kilometres from the Sun's surface to Earth?

About 8 minutes pass before light leaving the Sun arrives here. Deep inside the Sun, that same energy spent at least 10,000 years working its way out from the core.

Which farm animals are most commonly grazed beneath solar farms to keep the vegetation trimmed?

Sheep are the usual choice because they can graze among ordinary solar arrays with no changes to the design. In return for the mowing they get forage and shade.

Which physicist won the 1921 Nobel Prize for explaining how light knocks electrons out of a metal?

Albert Einstein won for his 1905 paper on the photoelectric effect, not for relativity. The same physics, light freeing electrons, is what lets a solar cell produce a current.

Which tiny state roofed its main audience hall with 2,400 solar panels in 2008?

Vatican City put the array on the Paul VI Audience Hall, a gift from a German manufacturer. It was sized to cover the building's heating, cooling and lighting all year.

Which carmaker launched the Powerwall in 2015, a home battery that stores rooftop solar power for the evening?

Tesla announced the Powerwall in April 2015 as a way to bring its car-battery know-how into the home. Only 500 pilot units were built that first year.

Which process in the Sun's core, turning hydrogen into helium, releases the energy that solar panels collect?

Nuclear fusion forces atomic nuclei together under crushing heat and pressure. The Sun is only about halfway through the stage of its life in which it shines this way.

In the Northern Hemisphere, which way should a fixed solar panel face to gather the most sunlight?

South is the rule above the equator, while panels in the other hemisphere point the opposite way. Installers then set the tilt according to the site's latitude.

Which element, the second most abundant in Earth's crust after oxygen, is used to make most solar cells?

Silicon makes up about 28% of the crust by mass, locked inside the minerals of ordinary rock and sand. Cells cut from its purified crystals dominate the world's solar market.

Which device turns the direct current from solar panels into the alternating current that homes use?

An inverter is needed because panels produce a steady one-way flow, while grid power reverses direction many times each second. Off-grid cabins running on batteries need one too.

At the 1878 Paris world's fair, Augustin Mouchot won a gold medal for using concentrated sunlight to make what?

Ice made with solar heat was the showpiece of his steam engine display. Cheap coal soon made the idea look pointless, and his research went no further.

Which American research lab unveiled the first practical solar cell in 1954?

Bell Labs credits the cell to Daryl Chapin, Calvin Fuller and Gerald Pearson. The same lab had already given the world the transistor.

Which country hosts the World Solar Challenge, a 3,000 km race for sun-powered cars first run in 1987?

Australia's race crosses the outback from Darwin to Adelaide. The first winner, General Motors' Sunraycer, averaged just 67 km/h, and by 2005 the pace had reached 100 km/h.

Which giant planet did NASA's Juno probe reach in 2016, the first mission there to run on solar panels?

Jupiter gets only 4% of the sunlight that Earth does, so every earlier mission to the planet carried a plutonium-fuelled generator instead.

Which country overtook Germany in 2015 to become the world's largest producer of solar electricity?

China took the lead in 2015 and just two years later became the first country to pass 100 gigawatts of installed solar panels.

Which 1958 American satellite, the first to carry solar cells, is the oldest human-made object still in orbit?

Vanguard 1 is so small that Nikita Khrushchev mocked it as the grapefruit satellite. Its sun-powered transmitter kept sending signals for more than six years.

Which natural process lets green plants store the Sun's energy as chemical fuel, the origin of wood and coal?

Photosynthesis locks away about 3,000 exajoules of sunlight in plant matter every year. Fossil fuels are that same harvest from the distant past, buried and compressed.

Which colour is the absorber plate in a solar water heater usually painted so it soaks up the most sunlight?

Black enamel paint is a common coating for the plate, which sits under glass with the water pipes embedded in it. Dark surfaces absorb the sunlight that pale ones bounce away.

Which particles strike a solar cell and hand their energy to electrons inside it, starting a current?

Photons give up their energy to electrons, and the cell's built-in electric field then sweeps those freed electrons off in one direction.

Which battery chemistry, the usual choice for home solar storage, won its developers a 2019 Nobel Prize?

Lithium-ion cells won the chemistry prize for John Goodenough, Stanley Whittingham and Akira Yoshino. They suit home storage because they pack a lot of energy into a small space and survive many charge cycles.

In 2016 the Swiss aircraft Solar Impulse 2 became the first piloted solar plane to do what?

To fly around the world on sunlight alone took Solar Impulse 2 more than 16 months, covering about 42,000 km in stages from Abu Dhabi and back.

Which small NASA helicopter, recharged by a solar panel above its rotors, made the first powered flight on Mars?

Ingenuity first rose from Mars in April 2021, hovering 3 metres up for about 40 seconds. A weak winter Sun later left it so short of power that it shut down completely at night.

Which physicist's name is given to a hypothetical shell built around a star to capture its energy output?

Freeman Dyson set out the idea in a 1960 paper on searching for artificial sources of infrared radiation. His inspiration was Olaf Stapledon's 1937 novel Star Maker.

The Desertec Foundation, set up in 2009, proposed helping to power Europe with solar farms in which desert?

The Sahara was chosen because parts of it see bright sunshine for up to 97% of daylight time. Studies suggested its sun could also run desalination plants for the region.

The Sharp EL-8026 and Royal Solar 1 of the late 1970s were early sun-powered versions of which gadget?

Calculator makers paired solar cells with liquid crystal displays, which need very little power. Some cheap modern models carry only a dummy panel and really run on a battery.

Which physicist discovered the photovoltaic effect in 1839, aged 19, in his father's laboratory?

Edmond Becquerel made his find with a simple electrochemical cell. Science ran in the family: his son Henri went on to discover radioactivity.

In which country did American inventor Frank Shuman build the world's first solar thermal power station in 1913?

Egypt's plant at Maadi used rows of mirrored troughs to drive an engine that pumped Nile water onto nearby cotton fields. The outbreak of the First World War then stalled the technology for decades.

Charles Fritts built an early solar cell in 1883 by coating which element with a thin layer of gold?

Selenium gave Fritts a working device, but it turned only about 1% of the light falling on it into electricity.

What are the sun-tracking mirrors called that reflect sunlight onto the receiver of a solar power tower?

Heliostats are turned by computer-controlled motors so that, as the Sun crosses the sky, each one keeps its reflected beam fixed on the same target.

Which heat-storing material lets many solar thermal plants keep generating electricity long after sunset?

Molten salt, typically a blend of sodium and potassium nitrates, stores the day's heat until it is needed. The Solar Two test plant proved the approach in the 1990s.

Which country built the vast Noor solar complex near Ouarzazate, named with the Arabic word for light?

Morocco's Noor complex ranks among the largest concentrated solar plants on Earth, with a field of some two million mirrors. It stands near the ancient fortified town of Ait-Ben-Haddou.

Which country built Bhadla Solar Park, whose panels cover 56 square kilometres of the Thar Desert?

India sited the park in Rajasthan, where it grew to a capacity of 2,245 megawatts, one of the biggest solar parks anywhere.

Which billing arrangement lets a home's surplus solar power offset the grid electricity it draws at other times?

Net metering banks unused electricity as credits, so July's sunshine can pay for December's lighting. Minnesota is usually cited as passing the first such law, in 1983.

Which Mediterranean country has required solar water heaters on nearly all new homes since 1980?

Israel passed the rule after the 1970s energy crisis, exempting only towers with too little roof. Its first commercial maker of such heaters had started up back in 1953.

Which author's 1941 short story "Reason" imagined a space station beaming the Sun's energy to the planets?

Isaac Asimov wrote it decades before engineers took up the idea: Peter Glaser proposed a real solar power satellite in 1968 and patented a microwave beaming method in 1973.

Which bird names the curve that charts California's grid demand sagging at midday as solar power floods in?

Duck is the outline the graph traces: a sagging belly at midday and a steep neck around sunset, when other power plants must ramp up fast to replace the fading Sun.

Which space station lost a main solar wing during its 1973 launch and had to be repaired by astronauts in orbit?

Skylab's meteoroid shield tore away during the climb, taking one solar array with it and jamming the other. The first crew freed the stuck wing and rigged a replacement sunshade.

What name is given to using the same land for both solar panels and crops or grazing animals?

Agrivoltaics got its name in 2011 and is also known as dual-use solar. The pairing is reported to pay off best in arid regions.

Which crystal structure, named after a Russian mineralogist, is used in a fast-improving class of solar cells?

Perovskite cells leapt from 3.8% efficiency in 2009 to about 27% by 2025. The mineral behind the name was first found in Russia's Ural mountains in 1839.

Which toxic heavy metal is paired with tellurium in a widely used type of thin-film solar panel?

Cadmium is a waste product of zinc refining, so panel makers use up a metal that already exists. Bound to tellurium it forms a stable crystal far less toxic than the bare metal.

Which element is commonly added to one layer of a solar cell to create its positive, electron-poor side?

Boron has one fewer outer electron than the atoms around it, leaving gaps that behave like positive charges. The opposite layer is dosed with phosphorus to add spare electrons.

Which two physicists give their names to the roughly 33% efficiency ceiling of a single-junction solar cell?

Shockley and Queisser calculated the limit in 1961. Cells that stack several junctions can beat it, with a theoretical ceiling of 86% for an endless stack under concentrated sunlight.

Which passive solar feature pairs thick, dark masonry with an outer sheet of glass to store heat for the night?

A Trombe wall is named after Félix Trombe, who did much to develop the design. The principle is far older: thick adobe and stone walls have long soaked up sunshine to warm homes after dark.

Which scientist, nicknamed the Sun Queen, helped create the solar-heated Dover Sun House?

Mária Telkes also built a sun-powered distiller during the Second World War that turned seawater into fresh water, saving the lives of downed airmen and torpedoed sailors.

Which rule, named after SunPower's founder, says solar module prices fall about 20% whenever shipments double?

Swanson's law, often likened to Moore's law for chips, honours Richard Swanson. By its measure the price of crystalline cells fell from about $77 a watt in 1977 to 36 cents in 2014.

Which Polish chemist found a crystal-growing method used for solar cells after dipping a pen in melted tin?

Jan Czochralski meant to dip the pen in his inkwell, and instead pulled out a thin metal thread that turned out to be a single crystal.