“Optimize Series Resistance to Match EIR” Algorithm

This algorithm finds the optimal series resistance value (with all other single-diode parameters constant) that generates a model-calculated effective irradiance response (EIR) close to some target (user-specified) EIR. In this way, the algorithm “tunes” a module’s series resistance to a desired EIR.


  1. Determine the direction that series resistance should increment.

Starting from Rs,max/2, series resistance is incremented once in both the positive and negative directions. The relative efficiencies at 800, 600, 400, and 200 W/m2 are calculated for each of the three series resistances: Rs,max/2 – increment, Rs,max/2, Rs,max/2 + increment (see “Module File Generation: Effective Irradiance Response (EIR) Calculation”). The error between each of the three generated EIR’s and the target EIR is calculated using a weighted RMS error:

where, for example,

If adding an increment to Rs,max/2 decreased the error, then the algorithm will increment in the positive direction (and vice versa).

  1. Iteratively find the optimal value of series resistance that generates the closest EIR to the target EIR.

The algorithm then increments Rs in the direction determined in step 1, calculating EIR error at the end of each iteration using the equation in step 1. It does this until the EIR error begins to increase again, Rs reaches Rs,max (in the case that the increment is positive), or Rs reaches 0.05Rs,max (in the case that the increment is negative). Using that final value of Rs, the following single diode parameters are recalculated: I0, γ, and μγ.