Why do I have different results in overpower loss than those specified during sizing ?

[André Mermoud]

The sizing tool (available by the button "Show sizing" when defining the system), is based on a simplified calculation of the Array power distribution, which is computed at an hypothetic temperature and doesn't include all losses of the system. Therefore the loss of the simulation (which is the reference) is usually lower that the loss at the design time.

Sometimes you can also have significantly higher (unexpected) losses during the simulation; this arises when the voltage of the overpower point is greater than the inverter's maximum Vmpp voltage. This situation is not taken into account in the pre-sizing tool (where we don't have a reliable value of the array voltage).

Now when your definition of the sub-array implies different numbers of strings on the inverter's inputs (i.e. number of strings not divisible by the number of MPPT inputs):

- In the version 5, the calculation was done globally for the whole sub-array, and the number of strings was averaged (for example 21 strings on 6 inverters: the simulation assuned 3.5 strings on each inverter.

=> the sizing result for overpower is coherent with the simulation.

- In the version 6, the calculation is performed for each inverter independently, and some inverters may operate in overpower conditions.

With our example: 3 inverters with 3 strings, and 3 inverters with 4 strings: in this particular case, the PNom ratio is 1.19 for the average (quite acceptable), but 1.03 for 3 strings and 1.36 for 4 strings. The overpower loss on the second case may be significamt.