I’ll try and give you an idea of how and why certain figures are relevant in the calculations.
Exceeding the voltage breaks down the insulation, which does irreparable damage.
So 250V is a hard limit. Open circuit voltage is what the MPPT will receive if it is switched on a cold day. So we will use the Voc we can expect on the coldest day in Durban in the calculations.
However, in terms of currents and wattages, we are talking about expected running conditions.
Those running conditions are stated in terms of “NOCT”, (normal operating cell temperature).
This doesn’t suit the marketing hype who would rather quote “STC” ( standard test conditions) values.
In reality, though your 555W panels, will only deliver that amount in a lab, they will deliver nowhere near that on a hot roof in Durban.
So to begin, the 250V capability of the MPPT should never be exceeded by the open circuit voltage of the PV string. But that isn’t the Voc as listed. That is the Voc on the coldest day in Durban.
So there is a Voc listed at a certain temperature, and a Voc temperature coefficient is also listed.
(I’ll use figures for a Trina 555W panel, I found on the web, you should check they are in line with your intended panels).
So, the Voc is listed as 38.1 V at 25deg C, ( with a 3% tolerance), which means it could potentially be 1.03 x 38.1V = 39.243V.
Then the temp. Coefficient of Voc is -0.25%/deg C and the coldest day ever recorded in Durban is 6 deg C, so we’ll use 5 degrees C as a safety margin.
This means that the panels could potentially operate at 25 - 5 = 20 degrees lower, and that would raise the coldest day Voc up by 20 x .25% = 5 %.
So 39.243V X 1.05 = 41.2052V.
As you can’t exceed 250V, the largest series string you can use is 250/41.2052 = 6.06 panels
So you could have 6 of those panels in a series string in Durban,( you wouldn’t get away with it in JHB as it gets colder there).
The next thing is the MC4 solar PV connections are only rated for 30A. If you tried to put 76A through them they would melt. Further to this 6mm copper cable is the largest cable that fits into these connections. Again this cable rating has to be respected.
For this calculation, you do not use Isc (@STC) because your MPPT will never see this current in reality.
It will see Impp(@NOCT) which is lower. This represents a truer reflection of what the panels will actually deliver. This is listed as 14.23 A. Bear in mind you probably will only get this occasionally as well.
So doubling up on that is still less than 30A.
So in terms of your array, you can have two parallel strings of 6 panels in series. This array will be the most efficient for a 250V MPPT.
To calculate an optimistic expectation of the power you can expect from such an array use Pmax Wp (@NOCT), which is 420W /panel.
12 X 420 W = 5040 W.
This means a 250/100 Mpp will be capable at a battery voltage of 51V.
(51V is for a 15-cell battery, you could even get more leeway using a 16-cell or higher battery).
Empirically, I have satisfied myself that if you are not using ideal North facing arrays, you would probably get away with a 250/85, or even maybe a 250/70.
I want to go on to say, that this size array is still hopelessly under-sized for a 2 x 5kW inverter capability. You want (at least) three of this sized array to achieve a balanced system.