question

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daver-1 asked

Mppt with parallel, series, or combined string. which is more efficient

I am thinking about a new setup.

It would be a singe phase multiplus II 48/5000 with a EM-24 meter with fase-compensation and one or more mppt's controlled by a cerbo GX

on the roof i would like to reuse the 8 450Wp 50v panels. I was thinking to make 2 photovoltaic strings of 1800Wp with there own mppt as each of those strings have a different orientation.

While using the victron mppt calculator, i was wondering if there is any reason to choose a "4panel series string" over a "2panel series & 2panel parallel string"

The only thing that i know that matters is to make sure that the string voltage is at least 5V above the battery voltage.

the advantage of the series string is that it can make use of thinner cabling but the disadvantage is that the mppt will be more expensive.

the advantage of the combined series&parallel string is that the mppt will be less expensive but the disadvantage is that there will be thicker cabling and probably a solar combiner box needed, as the string will probably exceed the maximum fuse rating of the panels.

Is one of them more efficient than the other, or is it just financially balancing the scale, before making a choice.

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2 Answers
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sharpener answered ·

You are correct when you say <the advantage of the series string is that it can make use of thinner cabling but the disadvantage is that the mppt will be more expensive. >

I think the difference between the efficiency of the various MPPTs is small compared with the potential losses in the cable.

So it really depends on the length of the cable run. I have just installed 8 more panels as 2 strings of 4 each with its own pair of cables. This was a compromise between reducing the losses over a distance of 25m and paying more for a 450V MPPT.

One installer proposed 4 strings of 2 but it would have needed 16mm^2 cable to meet the 3% voltage drop limit.


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jetlag answered ·

In my opinion you already figured it out.

The only thing I would add here is, that sometimes the available space for the PV panles might decide which MPPT you should use.

I had the same topic some month ago when decided to go with 8x 460Wp panels. With the panel voltage there is only the possibility to either go with a 4S2P or a 2S4P setup. As for both setups a combiner Box is needed anyway, the 2S4P with the much cheaper 150/70 made the deal instead of the 250/xx. The cable I need (20m x 16mm²) is not that much more expansive than a 10mm² I would have been using anyway. In both cases you have to have blocks with at least 2 panels in series that have to deliver the voltage to still enable battery charging. So shading should not make any difference.

Hope this is helpful for you. ;-)

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