Weather and irradiance data are used as input to PV performance models.  These data are directly measured, derived from measured data, or simulated using a stochastic model.

Irradiance is to power as insolation is to energy.  Or in other words:

• Irradiance is an instantaneous measurement of solar power over some area.  The units of irradiance are watts per square meter.  For practical purposes of measurement and interpretation, irradiance is expressed and separated into different components.
• Insolation is a measurement of the cumulative energy measured over some area for a defined period of time (e.g., annual, monthly, daily, etc.).  The common unit of insolation is kilowatt hours per square meter.  For these units to be interpreted correctly the time interval must be clearly stated (e.g., kW-hr per square meter annual insolation).

For each measurement of irradiance, a collection plane must be defined. For example, the collection plane may be oriented normal to the sun, or the collection plane may be horizontal to the surface of the Earth. The collection plane may also change its position such as terrestrial (inside the atmosphere) or extraterrestrial (outside the atmosphere). Note that the lack of a specifier generally indicates a terrestrial measurement. All extraterrestrial irradiance is direct due to lack of an atmosphere to cause scattering.

Irradiance measurements may also be classified by the portion of sunlight which is being measured. For example, some measurements may only measure direct irradiance, others may measure only diffuse irradiance, and others may measure both direct and diffuse (sometimes called “total”) irradiance.

Finally, irradiance measurements may indicate the field of view of the measurement instrument. In general, measurements of direct irradiance use a 5° field of view. Irradiance measurements may also be taken with an instrument which uses a 180° field of view (sometimes called “global”).

When these concepts are combined, it is possible to define irradiance measurements through a series of identifiers, for example: direct normal irradiance ($${DNI}$$), extraterrestrial irradiance ($${E_a}$$), diffuse horizontal irradiance ($${DHI}$$), global horizontal irradiance ($${GHI}$$).