[werk_ag]

I believe to give "full scale" with the 1.1 volts nominal reference voltage of the Nano needs a gain of 3.667, which I calculate requires 3.75 kohms.  With the wiper at "end of track" (5 kohms) the gain should be close to 3 to give a "full scale" (limiting) ADC reading at around 1833 Watt/m2 (I don't know what scale factor the software uses).

Hello Alan

Thank you for sharing your thoughts on this matter.
Can you please give me some light about how you did your calculation?
Perhaps I'm wrong, but I believe that to get the full scale from the SP-110 the variable resistor should be 4666 Ohm

http://mustcalculate.com/electronics/non...=&r2=10000
   

On the solar interface R1 is R2 and R2 is R1

Same calc from other source
http://yano.wasteonline.net/electronics/...=&rf=10000

   

On the solar interface Rf is R1 and Rg is R2