Science & history · Light and energy
How much light does the Sun give out?
Two numbers describe the Sun's output: its total power, and the share of it that crosses one square metre at Earth's distance.
Sunlight at Earth's distance today
Worked out in your browser for today's date.
Luminosity and irradiance
The IAU's nominal values are a solar luminosity of 3.828 × 10²⁶ watts and a total solar irradiance of 1361 W/m². Luminosity is everything the Sun radiates, in every direction. Irradiance is what arrives on a square metre facing the Sun at one astronomical unit, above the atmosphere.
Irradiance falls off with the square of distance. The page applies that rule to today's distance (method); it is a calculation for the top of the atmosphere, not a measurement at your window, where air, cloud, the Sun's height and the angle of the surface all reduce it.
Not the same as UV
The UV Index is a different quantity: a forecast of the risk of overexposure to ultraviolet, which depends on ozone, cloud, elevation and the time of day. A bright day and a high UV day are not the same thing.
Sources
- International Astronomical Union IAU 2015 Resolution B3: nominal solar and planetary conversion constants (Mamajek et al.) (opens in a new tab) for nominal radius 695,700 km, irradiance 1361 W/m², luminosity 3.828×10²⁶ W, effective temperature 5772 K · checked 2 October 2026
- Cool Moon Facts How the Sun pages calculate for the engine, its definitions, its independent checks and its limits
- US Environmental Protection Agency Learn About the UV Index (opens in a new tab) for UV Index scale and what changes it (elevation ~6% per km, clouds, ozone, solar noon) · checked 2 October 2026