Solar irradiance is the intensity with which radiation enters Earth’s atmosphere. An relatable way to think about solar irradiance is by looking at the difference between a 20-watt light bulb and a 100-watt light bulb. Both produce visible light in the same wavelengths, but the brightness and intensity are very different. The 100-watt bulb has a higher intensity, or irradiance. Solar irradiance is the amount of radiant flux on an area, and is measured in watts per meter squared (W/m²)⁹.

Source of the quote and the picture: http://www.fondriest.com/environmental-measurements/parameters/weather/photosynthetically-active-radiation/