Thermal Losses--Efficiency Value

[Abs_Zer_Res]

I'm trying to clarify my understanding of the relationship between the thermal balance equation and module efficiency. The equation is described as "an energy balance between ambient temperature and cell's heating up due to incident irradiance."

I realize that this is meant as an approximation (e.g. wind effects, etc.), and that the entered efficiency value is only meant to be used as a slight correction. But my impression is that module efficiency itself is dependent on irradiance level and module temperature. So I'm having trouble understanding how a constant efficiency value can be used to calculate thermal losses... when the efficiency itself depends on these factors?