Radiation, Irradiance, and Irradiation 

The term radiation is a general term used in discussions related to solar, but there are two specific terms or units which are used to distinguish between solar radiation power (irradiance) and

solar radiation energy (irradiation). Let us define-

Two radiation terms, irradiance and irradiation or insolation

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Irradiance: Irradiance (power density) refers to the rate of energy received by a surface per unit area; essentially irradiance is nothing but the flux of solar energy. Therefore, irradiance has the unit of J/m²-s or W/m².  

Irradiation or Insolation: It (energy density) is the amount of energy received by a surface over a given period of time. Thus, it is the integrated irradiance over a time. The unit of insolation is W-Hr/m² per hour or per day or per month. Sometimes the long-term average intensity of incoming solar radiation is given in units such as W/m² and called insolation with the duration (such as daily, monthly, annual, etc.) stated or only implied.