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tobyfw asked

Upgrading - swap MPPT 150/100 to 250/100 or get 2 additional 100/20 ?

Dear community,

I want to add four 590W panels to the roof. Before there were already 12 of them, 3s4p. Would you advice that I swap my MPPT150/100 for an MPPT250/100, to create a 4s4p, or instead get 2 additional MPPT100/20 with 2s1p each? All panels have the same orientation, but the shadow situation is complicated.

Pro buying two small MPPTs (instead of a Smart Solar MPPT250/100)
+ Having more MPPTs could be a benefit when there are difficult shadows
+ I understand that the maximum output of MPPT150/100 and MPPT250/100 are actually the same (5800W at 48V), so having three MPPTs would increase the theoretical maximum peak
+ Probably cheaper than selling the used MPPT150/100 and getting a new one

Against buying two small MPPTs (and for getting a MPPT250/100)
- adding one of the panel to each paralleled string would also help with the shadows, no big advantage of extra MPPTs
- having separate MPPTs means longer cables at lower voltages (2 more strings with 2 panels with Impp=17,25A open Voc=41,2V each) - thus, higher loss
- MPPTs might switch on a bit later, when only two panels in series

Any recommendations?
Thank you


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seb71 avatar image seb71 commented ·

Are the additional PV panels exactlly the same make and model as the old ones?

You wrote about shadow issues, but without details. Assuming the string voltage would be safe (should be if the new panels are the same as the old panels and they are presumably safe for your existing 150/100), why not get just another Smart Solar 150/xx with 3s1p or 3s2p (if you have space and money for 6 panels)?

100/xx models are to limiting for future upgrades.

Do you have a GX device in that system?

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tobyfw avatar image tobyfw seb71 commented ·

The four new panels are of the same series, but 10W more. RSM120-8-590 vs RSM120-8-600 (datasheet https://www.enfsolar.com/pv/panel-datasheet/crystalline/47544)

I understand that it would not matter if I add one 600W per string, as voltage is just summed up within the series, and all parallel strings would have the same voltage. The 600W panel would likely get 17.2A max like the 590W, instead of their nominal max of 17.3 A, but I do not think that is a big deal.

There is no more space on the roof (at least not with this orientation), so it is really a question of what to do with those 4 extra panels. If I put any more panels to different roofs, I understand I definitely should get a new MPPT for that.

Adding to that, I cannot add new parallel strings, because the remaining cables to the basement are 6mm2, which is ok for the 20A, but not for higher amperages. The 3s4p MPPT 150/100 is connected with 35mm2 cables from the distribution box.

The shadow situation is complicated, as we are in a city center, there is a tree moving through in the morning (we are not going to cut it down) and some neighboring buildings casting shadows in the evening.

Two panels will likely get more sun in the morning, the other two more likely in the evening. They are filling gaps at opposite ends of the roof.

I am using a Cerbo GX, Pylontech BMS with Quattro II

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seb71 avatar image seb71 tobyfw commented ·

There is no more space on the roof (at least not with this orientation)

So other orientations would be possible? If the main array (existing one) is facing South, adding a West or East facing array would be a great option. With a separate MPPT, a different orientation is not an issue.


The shadow situation is complicated, as we are in a city center, there is a tree moving through in the morning (we are not going to cut it down) and some neighboring buildings casting shadows in the evening.

Obviously no shadow at all is ideal, but if that's not possible, try to configure/re-configure the strings so as most of the time the shadow affects only panels from the same string (not from different strings at once), if you can.

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tobyfw avatar image tobyfw seb71 commented ·

Yes, I understand that with more MPPTs I can put panels on other roofs. But right now I am unsure what to do with the four remaining panels _on this roof_. If it is better to add two smaller MPPTs or swap the big one and extend the existing strings.

I do not see that getting an additional big MPPT (like 150/...) would help with that, as all panels within one MPPT string should have the same orientation (as far as I understand), with the possible exception of West-East roofs.


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seb71 avatar image seb71 tobyfw commented ·
So you already bought those 4 new panels?


The idea with a 150/xx was to have only one additional MPPT. And possibly add 6 more panels, not just 4.

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tobyfw avatar image tobyfw seb71 commented ·

Yes, the panels are already mounted, but not connected. I think you are suggesting something like the 150/45 (for up to 3s2p)

It would be an option, if I pair the existing spare cables 4x6mm2 into 2x12mm2, it should probably be fine, but:

As I wrote, I have space only for 4 more panels, so I am not sure about the benefits (two 100/20 are cheaper, and will get me one extra MPPT)



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seb71 avatar image seb71 tobyfw commented ·

For each additional MPPT you should also need PV wires from the roof to the MPPT, fuses, a switch disconnector for each PV array, battery wires, space on the wall, free (unused) bolts on the busbar, a VE.Direct cable (or VE.Direct -USB cable and a powered USB hub if you have more than 3 VE.Direct devices connected to Cerbo). So it's not just the price of the MPPT.


But if the panels are already mounted on the roof, using another roof area (with possibly more than 4 panels) is not an option and also talking into account the shading issues, then go for those two additional SmartSolar chargers (with 2 panels on each).

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