Solar insolation is the amount of electromagnetic energy (solar radiation) incident on the surface of the earth. This refers to the amount of sunlight shining down on the area under consideration. The values are generally expressed in kWh/m2/day. This is the amount of solar energy that strikes a square meter of the earth’s surface in a single day. This value is averaged to account for differences in the day’s length.
Solar Insolation – Definition, Glossary, Details – Solar Mangoadmin