In the results, the winning architecture is the following: 

PV – 339kW 

1MLI – 4,00 

Battery Converter – 598kW 

If we add a battery converter, does the model only consider iterations of this converter size? In other words, if we use a Sungrow 500kVA inverter, should we not only see options with multiples of 500kVA? Or does it mean that the optimal converter size is actually 598kW? 

This depends on which sizing feature is used. If the Search Space is enabled, HOMER will only consider the values entered in the table. Alternatively, if the HOMER Optimizer is enabled, HOMER will consider every converter size between the upper and lower limit. More information can be found in the User Manual.