Effective irradiance is total plane of array (POA) irradiance adjusted for angle of incidence losses, soiling, and spectral mismatch. In a general sense it can be thought of as the irradiance that is “available” to the PV array for power conversion.

In the context of the Sandia PV Array Performance Model (SAPM), effective irradiance ($E_{e}$) is defined specifically as:

$E_{e}=\frac{I_{sc}}{I_{sc0}\left&space;\{&space;1+\alpha&space;_{Isc}\left&space;(&space;T_{c}-T_{0}&space;\right&space;)&space;\right&space;\}}$, where $I_{sc}$ is the short circuit current.  It can be calculated from Eq. 1 described in the SAPM, from measured irradiance, air mass, angle of incidence, and several empirically determined module coefficients.

Alternatively, effective irradiance can be measured directly by obtaining IV curves from a matched reference module.

$E_{e}=\frac{I_{scr}}{I_{sc0r}\left&space;\{&space;1+\alpha&space;_{Iscr}\left&space;(&space;T_{cr}-T_{0}&space;\right&space;)&space;\right&space;\}}\times&space;SF$

• $SF$ is the soiling factor (=1 when clean)

A simplified approach using a single irradiance sensor has also been suggested, shown below.  Caution should be used when applying this method because irradiance sensors (pyranometers) require careful calibration and frequent cleaning.  These calibrations may not account for all instrument responses (e.g., angular), and instruments can vary in their spectral response and acceptance angles, etc. (thermopile vs. photodiode)

$E_{e}=\frac{E_{POA}}{E_{0}}\times&space;SF$

• $E_{POA}$ is the plane of array irradiance.
• $E_{0}$  is the reference irradiance ($1000&space;W/m^{2}$)