Solar Constant (Total Solar Irradiance)
| Value | 1,361 W/m² |
| Status | Measured: ± 0.5 W/m² (0.00037 relative) |
| Source | Kopp & Lean (2011); TSIS-1 total solar irradiance monitoring |
| Categories | Astronomical |
| microwatt per square meter | 1,361,000,000 μW/m² |
| milliwatt per square meter | 1,361,000 mW/m² |
| microwatt per square centimeter | 136,100 μW/cm² |
| watt per square meter | 1,361 W/m² |
| BTU per hour square foot | 431.43473 BTU/(h·ft²) |
| milliwatt per square centimeter | 136.1 mW/cm² |
| langley per minute | 1.9517208 Ly/min |
| kilowatt per square meter | 1.361 kW/m² |
| watt per square centimeter | 0.1361 W/cm² |
| megawatt per square centimeter | 1.3610000e-07 MW/cm² |
Learning zone
Divide the Sun's luminosity by the area of a sphere one au in radius and you get 1361 watts on every square metre facing the Sun, measured above the atmosphere. It is misnamed twice over. It is not constant over the year — Earth's orbital eccentricity swings it by ±3.4 per cent, peaking in early January at perihelion — and it is not constant over the solar cycle either, varying about 1 W/m² between sunspot maximum and minimum. The accepted value itself moved: satellite radiometers reported 1365–1367 W/m² for decades until the TIM instrument on SORCE exposed a stray-light error in the older designs, revising it down to 1361 in 2011.
Averaged over the whole rotating planet the figure becomes 1361/4 ≈ 340 W/m² at the top of the atmosphere, of which roughly 30 per cent is reflected straight back — the numbers from which Earth's equilibrium temperature, and the size of the greenhouse effect, are derived.