Each PV device (e.g., cell, module, string) in operation will have a maximum power point ($I_{mp}$ and $V_{mp}$) for the ambient operating conditions of incident irradiance and device temperature. Even at the same irradiance and temperature, seemingly identical devices will not have the same maximum power point because of small manufacturing differences. However, when interconnected in series and parallel to form a DC array, series strings of PV devices all must carry the same current and parallel strings must operate at the same voltage measured at the point of common connection. This fact means that the individual devices are forced off of their maximum power point to a compromise value that gives the maximum power out of the devices as combined into the DC array.

The total impact of mismatch losses may be calculated as follows:

$Mismatch&space;Loss&space;=&space;1-\frac{DC&space;Array&space;Output}{\sum&space;Module&space;MPP&space;-&space;\sum&space;DC&space;Wiring&space;Losses}$

However, mismatch due to heterogeneous irradiance (shading) is often modeled separately from mismatch due to variations in module performance or temperature.