DNI represents direct sunlight falling perpendicularly on a surface, such as a solar panel. DHI is diffuse sunlight that has been scattered in the atmosphere and arrives equally from all directions on a horizontal surface. GHI is the total shortwave radiation on a horizontal surface from both direct and diffuse sunlight, and is the most important parameter for calculating PV electricity yield. GTI represents irradiation on a tilted surface, such as a solar panel, and includes a small amount of ground-reflected radiation in addition to direct and diffuse components.