[AllyCat]

Hi,

You're not measuring the resistance between the "ends" of the pot are you?   It should be between the "wiper" (ground) and the end that is connected to the Op Amp input.  The value should be about 3.33 kohms to give a gain (amplification multiplier) of 4.  Increasing the resistance gives lower gain, which you possibly need because at 1500 w/m2 the SP-110 should give 300 mV output. 

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).

But all this should make no difference to the "zero" (night time) value.  Zero times 3.75 (or any value) should still be just zero!  So in the "dark" you need to discover what isn't zero, which with a good multimeter shouldn't be too difficult:  

Measure the voltages on pins 1, 2 and 3 of the Op Amp relative to a good ground (probably the PCB track, wiper of the pot, or ground connection of the SP-110) to a resolution of better than 1 mV.  Those three voltages (which could be measured on the two ends of R1 and the PV input) should give an indication whether it's the SP-110, the Op-Amp (input offset error), the Nano ADC (zero reference error) or something else that is "wrong".

Cheers,  Alan.