The Earth absorbs 3.86 million exajoules of sunlight a year

Every year the Earth soaks up about 3.86 million exajoules of sunlight, a truly massive amount. An exajoule is a billion billion joules, and the world’s entire energy supply in 2025 came to 655 of them⁴.

That sunlight arrives at the top of the atmosphere at about 340 watts on every square metre, averaged over day and night and the whole globe. Clouds, ice and air reflect about 100 of those watts straight back to space, and the planet’s land, oceans and air absorb the other 240¹². Spread over the Earth’s whole surface and the seconds in a year, that comes to the 3.86 million exajoules, by our own arithmetic.

Of the 240 watts the planet absorbs, the air takes about 80 and the land and oceans about 160. Slightly more, about 185 watts, reaches the ground, but about 25 of those bounce straight back up off snow, ice and other bright surfaces¹. These are averages over many years, not measurements of any one year, and the IPCC rates the figures at the ground as less certain than those measured from space¹.

An hour and a half of sunlight equals a year of the world’s energy

The sunlight the Earth absorbs in about 89 minutes matches all the energy the world used in 2025: every barrel of oil, tonne of coal, cubic metre of gas, nuclear reactor, dam and wind turbine together. Over a year it is about 5,900 times the world’s use. Both figures are our own arithmetic on the IPCC and IEA numbers¹⁴.

The popular version, that the sun delivers a year’s energy in about an hour, holds only for all the sunlight arriving at the top of the atmosphere, before any is reflected. On that measure it takes about 63 minutes.

That comparison is between sunlight and fuel, not between sunlight and what panels could deliver. The sun’s energy has to be caught and converted first, and that is where most of it is lost, as the sections below explain.

Almost every renewable energy resource runs on sunlight

The renewable energy resources are sunlight, wind, flowing water, plants, the oceans and the Earth’s own heat, and almost all of them are sunlight in another form. Sunlight dwarfs the rest: about 3.9 million exajoules a year, against about 6,000 for the wind and 147 for the world’s rivers, by the IPCC’s estimates of each resource’s natural flow⁹.

The links are direct. Close to half of the sunlight reaching the ground goes into evaporating water, which falls as rain and fills the rivers that turn hydropower turbines⁹. Uneven heating of the planet sets the air moving: the whole atmosphere generates about 900 terawatts of wind power, around 28,000 exajoules a year by our conversion, from about 2% of the energy the sun spends heating some parts of the Earth more than others¹⁰. And plants on land store about 1,260 exajoules a year in new growth above ground⁹.

Only two resources come from somewhere else. Geothermal energy is the Earth’s own heat, leaking out at about 47 terawatts¹¹, roughly 1,500 exajoules a year, and the tides are driven mainly by the pull of the Moon.

The resources differ enormously in size, and in how much of them people could use. The second column is what the IPCC calls technical potential: what could be produced with today’s technology once other demands on land are allowed for⁹⁷⁸.

The natural flows come from the IPCC’s 2011 special report on renewable energy (the plants row is its figure for growth above ground), and the technical potentials from its 2022 assessment. “The theoretical potential of RE is much greater than all of the energy that is used by all the economies on Earth,” the 2011 report concluded. “The challenge is to capture it”⁹.

Coal, oil and gas are sunlight stored for millions of years

There is far more energy in sunlight than in all the fossil fuels left. The Earth absorbs as much sunlight in about four days as all the coal, oil, gas and uranium that can profitably be extracted today, about 42,500 exajoules⁵.

Add every deposit geologists think exists but that is not yet proven or worth extracting, and the total is about 547,000 exajoules⁵⁶. That is about seven weeks of absorbed sunlight, or a seventh of one year’s worth. Both comparisons are our own arithmetic.

Fossil fuels are sunlight too, caught by plants long ago and buried, which is why they pack so much energy into so little space. A 2003 study worked out that the coal, oil and gas burned in 1997 came from ancient plant matter more than 400 times the weight of everything that grows on Earth in a year, and that a single US gallon of petrol took about 90 tonnes of it¹³.

Solar panels on about 1% of the world’s land could meet its energy needs

Only a sliver of the sunlight can be turned into useful energy. The IPCC puts what solar panels could produce, once competition for land is allowed for, at about 1,080 exajoules a year⁷. That is roughly double the world’s current energy use, but only about two and a half hours of the sunlight the Earth absorbs, by our own arithmetic.

Today’s silicon panels turn about 22.7% of the light that hits them into electricity¹⁷. That is around 45 times the half a percent that crops grown for biofuel manage⁹.

The land needed is smaller than most people guess. A 2024 study estimated that conventional solar panels on 0.5 to 1.2% of the world’s land could meet all the energy demand projected for 2085, about the area of today’s cities. Allowing for the effects of climate change raises that to 0.7 to 1.5%¹⁴. Farming, by comparison, already uses more than a third of all land¹⁵. The same study found that panels over the 0.7% of land covered by highways could, in its most optimistic scenario, supply 1.2 times the demand of 2085¹⁴.

Rooftops alone could in principle generate more than half of the world’s current electricity, though the share that is affordable and that local grids can take is only a small part of that¹⁶.

All the world’s solar panels capture about 80 seconds of sunlight a year

In 2025 the world’s solar panels generated 2,807 terawatt-hours of electricity, about 10 exajoules and about 11 times as much as in 2015¹⁸. That is as much energy as the Earth absorbs from the sun in about 80 seconds. All renewable electricity together, including hydropower and wind, comes to about five minutes’ worth. Both comparisons are our own arithmetic, and they set electricity produced against sunlight absorbed, so they show how little is captured, not how efficient panels are.

In 2020 the head of the International Energy Agency (IEA) made a bold call about solar.

The electricity it produces has grown by a record amount every year since: 335 terawatt-hours more in 2023, 491 more in 2024 and 646 more in 2025¹⁸. How large a share of the world’s electricity that now is, and what it is replacing, has its own page.

Sunlight is spread thin, and that is the case against an all-renewable grid

Sunlight and wind are abundant but spread thin. A review of published estimates for the United States found that non-renewable power, fossil fuels and nuclear, produces about a thousand times more power per square metre of land than renewable power does²¹. That is why even the 1% of land in the estimate above is an area about the size of all the world’s cities. The energy scientist Vaclav Smil argues that the low density means rebuilding where and how energy is produced, not just swapping fuels²².

How far that limits an all-renewable system is disputed. In 2017, 21 researchers led by Christopher Clack challenged a plan to run the continental United States entirely on wind, water and sunlight, saying it used invalid modelling tools and implausible assumptions²³.

The plan’s authors, led by Mark Jacobson, replied in the same journal that every one of the errors claimed was “demonstrably false”²⁴. The dispute was about one country’s grid, not the world’s. In the same US review, solar was the only renewable source whose power per square metre had been rising over time²¹. And in a place as sunny as Sicily, a set of silicon solar panels pays back the energy used to make it in about a year and, over a 20-year life, returns about twenty times that energy¹⁷.

The Earth is absorbing more sunlight than it used to

The amount of sunlight the Earth absorbs is not fixed. The sun’s own output varies by only about a tenth of a percent over its 11-year cycle¹², so the bigger changes come from the Earth itself. From 2005 to 2019 the planet’s energy imbalance, the gap between the energy it absorbs and the heat it sends back to space, roughly doubled²⁶. The main causes were more sunlight being absorbed, because clouds and sea ice were reflecting less of it away, and greenhouse gases trapping more heat²⁵.

In 2023 the Earth’s reflectivity hit a record low, apparently because of fewer low clouds, and that extra absorbed sunlight helps explain why the year was so unexpectedly hot²⁷. The imbalance is only about 0.3% of the sunlight the Earth absorbs²⁵, so the yearly total barely moves, but it is all heat the planet keeps.

By 2050, solar panels could catch 17 minutes of the year’s sunlight

The IEA’s scenarios show how much more the world might capture. If today’s policies stay as they are, solar panels would generate about 17,000 terawatt-hours in 2050; under the policies governments have announced, about 20,400; and on a path to net zero emissions, about 35,500¹⁹. By our own arithmetic those equal about 8, 10 and 17 minutes of the sunlight the Earth absorbs in a year, and the net zero path would use about an eighth of the IPCC’s estimate of what solar panels could produce⁷.

Solar generation grew by 30% in 2025¹⁸. If it kept growing that fast, it would reach the net zero path’s 2050 level by about 2035, a rough extrapolation rather than a forecast, since growth rates usually slow as an industry gets bigger.

Even the most ambitious path leaves the sun’s supply almost untouched. That suggests the limits on solar are the land, materials, grids and money needed to catch it, and how fast the world is willing to build. How much of today’s fossil energy that building actually displaces is the question the article on whether solar is replacing coal takes up.