Solar constant
The solar constant is the average intensity with which solar radiation reaches the Earth. Intensity means power per area or energy per time and area. The solar constant is 1361 W/m2 or 1361 J/(s x m2).
As the position of the Sun and the Earth in relation to each other is constantly changing, the Sun's radiation is distributed over a rounded body and does not reach us vertically everywhere and the Earth's atmosphere also influences how much solar radiation reaches us, the following applies to the solar constant: the Sun and the Earth are at an average distance from each other, the Earth's atmosphere is ignored and the radiation reaches the Earth vertically.