If you use a reference solar cell then the provider has given a data sheet for calibrating. The cell you used is a reference cell or self-made one?

A way out is to measure incoming radiation on a horizontal plane with a reference pyranometer, as well as PV cell temperature, which is quite different from atmospheric temperature. Then try to catch the behavior of current and voltage or apparent power (product of Voltage and Current (DC)) on a time basis of a few minutes.

Calibrate using the measured incoming radiation of your pyranometer and the Power, Voltage and Apparent Current of your PV cell(s).

You could use a PV Current, Voltage or Power model as well. There are hundreds of them in the literature. They all boil down to a rather stable model scheme, when you study them a bit. Once you calibrated your model, you can invert it an get solar incoming radiation measured by your PV cells power output. You have to convert that to incoming radiation on a horizontal using a bit of trogoniometry.

Mind that the relationship between PV power output and incoming radiation is non-linear due to the rather strong temperature dependency of PV cell(s).

PV cell(s) become less responsive to radiation with increasing temperature. Lucky enough even at high temperatures PV cells still produce electric Power. They are remarkably enough most efficient when it is cold and cloudless, hence in a cold desert. Temperature dependencies of Current and Voltages can be found on the spc sheets of the producers of solar panels. They are freely available (on the web as well).

So you are on your way.

Lots of luck ion your way to the Sun.

Frank

The solar cell would have to be calibrated against a reference pyranometer _before _deployment.