When we connect PV panels we connect them in series. Like when you add voltages to a battery by connecting them in series, the thing happens with PV panels.

There are two enviromental variables that affect the output votlage of a panel:

• solar irradience
• temperature

The first one is obvious, the more sun the more power (well voltage, but same thing here). The second one is less obvious, but as the temperature drops the more voltage the panel will produce.

This form accounts for the voltage with the beta field. This is typically negative, and if you aren't sure what the value is, it willl be on the PV panel cutsheet, or you can use -0.5 to start with.

The temperature should be the minimum daytime temperature that you expect for this panel. For example, if you are in a northern climate it may be -25C and in a more temperate climate it may be 25C.