Solar Irradiation

Solar irradiation measures the total solar radiant energy received by a surface per unit area over a specified period. It is obtained by integrating solar irradiance over time and is commonly expressed in kilowatt-hours per square metre per day or year.

What site measurements show

Solar irradiation describes the solar-energy resource available at a proposed photovoltaic site. Measurements may refer to a horizontal surface or to the actual plane of the modules.

• Global Horizontal Irradiation (GHI) is the total direct and diffuse solar energy received on a horizontal surface.
• Direct Normal Irradiation (DNI) measures direct-beam solar energy on a surface kept perpendicular to the Sun's rays.
• Diffuse Horizontal Irradiation (DHI) measures sunlight scattered by the atmosphere and received on a horizontal surface.
• Plane-of-array irradiation represents the energy incident on modules at their chosen tilt and orientation.

Use in photovoltaic site evaluation

Higher irradiation generally increases the potential electricity generation from a photovoltaic array of a given capacity. Long-term monthly and annual data reveal seasonal variation and help estimate energy yield, select module orientation and tilt, and compare candidate sites.

• Irradiation varies with latitude, season, cloud cover, atmospheric conditions and shading.
• It measures incident solar energy, not electricity output; actual generation also depends on module efficiency, temperature, system losses and availability.

Irradiation and irradiance

Solar irradiance is the instantaneous solar power falling per unit area, usually measured in watts per square metre. Solar irradiation is the corresponding energy accumulated over time, so the two terms should not be used interchangeably.