I have 2 Axperts connected in parallel.  12 x 330w panels connected to the Master, and the battery bank is connected to the Master as well.

I'm considering installing an additional 4 x 330w panels.  The question is:

Do I reconfigure the 12 existing ones to connect the 16 to the Master?  or do I reconfigure the 12 to connect 8 to the Master and 8 to the Slave? or to simplify the install effort now, just connect the 4 to the Slave?  What all do I need to consider?  Does the fact that the battery bank is connected to the master influence the decision I need to make?

I'm intending to use the system from Solar as much as possible during the day, utility during the evening, and battery as backup all the time.

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I have 2 Axperts connected in parallel.

But you didn't say what model, or how much PV power it is capable of using.

1 hour ago, Radar said:

12 x 330w panels connected to the Master, and the battery bank is connected to the Master as well.

Surely the battery is connected to the slave as well. Otherwise, it has no source of power other than utility.

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Do I reconfigure the 12 existing ones to connect the 16 to the Master? 

12 x 330 W is just under 4000 W, so I'll assume that your inverters are 4 or 5 kVA 48 V models that are capable of 4000 W or 4500 W of PV power. 16 x 330 = 5 280 W, which is too much for any Axpert model I know of.

1 hour ago, Radar said:

or to simplify the install effort now, just connect the 4 to the Slave?  What all do I need to consider?

The main consideration is whether there is partial shading over some group of panels, or some panels have a different orientation. As with any dual MPPT system, you should split the panels such that those with the same solar conditions (same shading or lack thereof, and same orientation) are on the same MPPT.

If the shading and orientation aren't markedly different, then the simpler option is the best.

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Does the fact that the battery bank is connected to the master influence the decision I need to make?

No, because it's imperative that both inverters are connected to the same battery.

5 hours ago, Coulomb said:

But you didn't say what model, or how much PV power it is capable of using.

Surely the battery is connected to the slave as well. Otherwise, it has no source of power other than utility.

12 x 330 W is just under 4000 W, so I'll assume that your inverters are 4 or 5 kVA 48 V models that are capable of 4000 W or 4500 W of PV power. 16 x 330 = 5 280 W, which is too much for any Axpert model I know of.

The main consideration is whether there is partial shading over some group of panels, or some panels have a different orientation. As with any dual MPPT system, you should split the panels such that those with the same solar conditions (same shading or lack thereof, and same orientation) are on the same MPPT.

If the shading and orientation aren't markedly different, then the simpler option is the best.

No, because it's imperative that both inverters are connected to the same battery.

Apologies.  They are both 5kva 48v models.  And yes the battery is connected to both of them (rookie mistake).

Ok, so the 16 panels would be too much for any one MPTT to manage.  Got it.

The ones currently connected are not all the same solar conditions (two types of orientation) and the additional 4 would be a third type of orientation. I'd need to match the 4 with closest in terms of solar condition then.

Does that mean splitting them across the two inverters (8 each) is most probably the safest?  If I use SOL mode, do they 'share' the cumulative power generated from the 16 panels, or does each inverter only use its own MPTT for its own load?

9 hours ago, Radar said:

Does that mean splitting them across the two inverters (8 each) is most probably the safest?  If I use SOL mode, do they 'share' the cumulative power generated from the 16 panels, or does each inverter only use its own MPTT for its own load?

No need to split the PVs evenly. Just as long as no one inverter has too much PV power (probably 4500 W for you). The panel power is shared by the battery cables, whether the power goes all the way to the battery or not.

Grouping by commonality of orientation and/or shading is more important. Since you have two orientations and now a third, adding the four new panels to the second inverter is probably nearly as good as it gets, and may be less work.