Is there need for a control set-up in this experiment?

[Nongsai]

Dear community,

Please let me know if this is the wrong category.

I have been working on a device enabling to characterize the Max Power Point (MPP) and Fill Factor (FF) of solar cells by applying various loads to a solar cell and extracting the current-voltage curve. Now it is time for the experiment itself.

The idea for the experiment is to determine the FF of, say 10 cells under real sun irradiation, under sunny conditions. The parameter which can (strongly?) influence the results is that the conditions of real sunlight irradiation are highly unsteady: the proportion of diffuse and direct irradiation can vary significantly within minutes or seconds, influenced even by clouds invisible to the human eye.

In order to cope with this, various sets of measurements will be performed and they will then be compared with linear regression methods.

My question to the engineers/scientists here is this: in this kind of experiment, is there necessity for a control setup? I mean, I'm comparing various sets of measurements in which nothing changes except the illumination radiation (assuming constant temperature since experiments are conducted around the same time of the same day). But these all seem to be experimental setups, I wouldn't know which one to define as the control setup.

Thanks a lot for any clarifications regarding this question!