What is the solar constant?

The solar constant (S₀) is the total solar irradiance at Earth's mean distance of 1 AU, measured above the atmosphere. Its current accepted value is approximately 1,361 W/m² (IAU 2015 / SORCE/TIM satellite). It varies by about ±0.1% over the 11-year solar cycle.

Why does irradiance follow the inverse-square law?

The Sun emits a total luminosity L☉ uniformly in all directions. At distance d, that energy is spread over a sphere of area 4π d². The power per unit area (irradiance) is therefore L☉ / (4π d²). Doubling the distance quarters the irradiance.

What value of solar luminosity does this tool use?

L☉ = 3.828 × 10²⁶ W, the IAU 2015 nominal solar luminosity. This is a fixed reference value and excludes neutrino emission. The actual luminosity changes by less than 0.1% over a solar cycle.

How does this differ from the solar flux at a planet's surface?

This calculator gives the flux at the top of the atmosphere (or in space) at that distance. Surface flux is reduced by atmospheric absorption, scattering, cloud cover, and the planet's obliquity and albedo. For example, Earth's mean surface insolation is roughly 240 W/m² after reflection and absorption losses.

What is 1 AU in kilometers?

1 AU (astronomical unit) = 149,597,870.7 km, the IAU 2012 definition. It closely matches Earth's mean orbital radius. Light travels this distance in about 8.317 minutes (499 seconds).

Solar Constant Calculator

Name: Solar Constant Calculator
Availability: InStock
Author: Nham Vu

Enter a distance from the Sun (in AU, km, or light-minutes) and get the solar irradiance in W/m² using the inverse-square law.

Distance from the Sun

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Summary

Enter a distance from the Sun (in AU, km, or light-minutes) and get the solar irradiance in W/m² using the inverse-square law.

How it works

Enter a distance from the Sun in AU, kilometers, or light-minutes using the unit selector.
The calculator converts your input to meters and applies the inverse-square law: S = L☉ / (4π d²).
The solar luminosity L☉ = 3.828 × 10²⁶ W is the IAU 2015 nominal value.
The result shows solar irradiance in W/m² and the ratio relative to Earth's value (1,361 W/m² at 1 AU).
A planet comparison table lets you select a preset solar system body to populate the distance field instantly.

Use cases

Calculate the solar power available to a spacecraft or satellite at a given orbit.
Determine how solar panel output changes as a probe travels from Earth to Mars or Jupiter.
Compare solar energy flux across the inner and outer planets for astronomy coursework.
Estimate the solar irradiance on an exoplanet at a known orbital distance.
Verify inverse-square law behavior for physics or astrophysics students.
Explore how much energy a Dyson sphere element would intercept at different radii.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu