There are a number of different units that are frequently used when talking about solar irradiance. PV engineers typically use W/m² for irradiance. Insolation (now commonly referred to as irradiation) differs from irradiance because of the inclusion of time. For instance, the number of Watts over a square meter for a period of time, typically a day, gives us the units kWh / m² / day.

A commonly used term is “peak sun hours,” which reflects the energy received during total daylight hours as defined by the equivalent number of hours it would take to reach that total energy value had solar irradiance averaged 1000 W/m². This term is interchangeable with kWh / m² /day.