# Sunlight: How energy resides

Why sunlight is huge in total but thin per square metre

Source page: https://www.materialinsights.org/energy/e_sunlight/resides/

What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.

Sunlight is energy carried as electromagnetic radiation, released by nuclear fusion in the Sun's core. It arrives at about 1,361 W/m2 above the atmosphere but averages only about 161 W/m2 absorbed at the ground, so collecting it takes large areas.
 Form Electromagnetic radiation (visible light, near-infrared and ultraviolet) Origin Direct solar

#### How it got there
 Hydrogen fusion in the Sun's core Radiation leaves the Sun's surface Light crosses 150 million km of space Part is reflected or absorbed by the atmosphere and clouds Remaining light reaches the ground

#### How concentrated it is

| Measure | Value | Note |
|---|---|---|
| **Total solar irradiance above the atmosphere** | 1,361 W/m2 (facing the Sun)^[1](https://earth.gsfc.nasa.gov/climate/projects/solar-irradiance/science) | TSIS-1 measured 1,361.6 plus or minus 0.3 W/m2 at the 2019 solar minimum. |
| **Global average input at the top of the atmosphere** | 340 W/m2^[1](https://earth.gsfc.nasa.gov/climate/projects/solar-irradiance/science) | One quarter of 1,361, because a sphere has four times the area of the disc it presents to the Sun. |
| **Absorbed at the Earth's surface, global mean** | 161 W/m2^[2](https://doc.rero.ch/record/321020?ln=en) | Day and night, all latitudes and all weather averaged; about 185 W/m2 reaches the surface before reflection. |
| **Clear-sky peak at the ground ('one sun')** | 1,000 W/m2^[3](https://energyeducation.ca/encyclopedia/Solar_energy_to_the_Earth) | The standard test condition (AM1.5) used to rate solar panels. |
| **Power density of operating solar farms** | 5.4 W of electricity per m2 of plant area^[4](https://www.hks.harvard.edu/publications/observation-based-solar-and-wind-power-capacity-factors-and-power-densities) | Mean of 1,150 US solar plants in 2016. |

In the Sun's core, hydrogen nuclei fuse into helium and the mass lost is released as energy. Over time this energy works its way to the surface and leaves as light. At Earth's distance it carries 1,361 W on every square metre facing the Sun. The sunlight that falls on land and sea also powers wind, waves, rain and plant growth, which is why most other renewable sources count as indirect solar.

The problem for collectors is how thinly the energy is spread. Night, the low winter sun, clouds and the atmosphere cut the global average absorbed at the surface to about 161 W/m2. A real solar farm, after conversion losses and the space between panel rows, delivers about 5.4 W of electricity per square metre of site. That is roughly ten times more than a US wind farm per unit of land. Capturing sunlight at scale therefore needs a lot of land or roof area, and some form of storage to cover nights and winters.

#### Sources

1. [Solar Irradiance: Science](https://earth.gsfc.nasa.gov/climate/projects/solar-irradiance/science) NASA Goddard Space Flight Center, 2024
2. [The global energy balance from a surface perspective (Wild et al., Climate Dynamics 40, 3107-3134)](https://doc.rero.ch/record/321020?ln=en) Springer / RERO DOC, 2013
3. [Solar energy to the Earth](https://energyeducation.ca/encyclopedia/Solar_energy_to_the_Earth) Energy Education, University of Calgary, 2024
4. [Observation-based solar and wind power capacity factors and power densities (Miller and Keith, Environmental Research Letters)](https://www.hks.harvard.edu/publications/observation-based-solar-and-wind-power-capacity-factors-and-power-densities) Harvard Kennedy School, 2018

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_sunlight/index.md)
- [Overview](https://www.materialinsights.org/energy/e_sunlight/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_sunlight/correlation/index.md)
- [Abundance](https://www.materialinsights.org/energy/e_sunlight/abundance/index.md)
- [Fluctuation in time](https://www.materialinsights.org/energy/e_sunlight/time/index.md)
- [Across the Earth](https://www.materialinsights.org/energy/e_sunlight/geography/index.md)
- [Impact on nature](https://www.materialinsights.org/energy/e_sunlight/nature/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_sunlight/links/index.md)