[werk_ag]

Hi, welcome

The voltage of the solar cells doesn't matter, as what we need to measure is the voltage drop at the shunt resistor (short circuit current), which should be as low as possible to keep the cell at the short circuit zone.

Increasing the number of cells will only make that a lot of power should be dissipated on the shunt resistor.

Maybe you would like to read the original study from Charles G. Wright, on which our solar cell pyranometer is based. Here is the link:  

http://chuck-wright.com/projects/pv-measure.html