Hi all,
I’ve perused and learnt from a few forum threads on panel string sizing and am busy planning my own setup, to link in with a 8KW Sunsynk and 2x 5KW batteries. 

I did pose one or two string-related questions to SunSynk support while clarifying something else, but they deferred to solar installers for answers saying it required an in situ inspection. I suspect they might not have understood what I was trying to clarify. 

With the 8KW inverter having 2x MPPT inputs (each with two strings), I’m trying to plan panel sizing and config given the sub-optimal roof space and direction. It is what it is…

The image below has some possibilities that I’m considering. Note that panel sizes in the mockup are not to scale viz. the roof size and are all meant to be the same spec, and merely illustrate aspect and possible quantities given other limitations.

I’d appreciate brief answers on some specific questions - apologies for the number - which are basically all relating to string size and config:

Assumption: The inverters specs seem to be that each MPPT input can handle 22A which I assume means that two different strings feeding into each MPPT can handle panels with an A rating of less than 22A, subject to a V limit of 450V (or a bit less to be safe). 

 

(1) In terms of 2 strings per MPPT input, can each string's panels have different angles, provided that the panels within the same string face very broadly the same way? IOW, can one string of MPPT 1 face one way, and the second string of MPPT 2 face another (as in mockup, panels in yellow and pink frame), provided that both should receive sun together at more or less the same time hence providing volts to the SunSynk?

(2) Does each of the two strings within one MPPT have to individually meet the threshold of 150V start-up / minimum voltage to get the inverter going? In my mockup below, this is especially referencing MPPT 1 (String 2, pink line), where there is most definitely only space for 2 panels, i.e. a combined 90V-100V. String 1 (red line) on the same MPPT would always receive sun at the same time, and presumably (since there is space for 4-5 panels) would easily have say 250V. I think the optimal voltage is around 370V or so, so does this calculation add the 2 panels (say 90V) to the 5 panels (say 225V) to give a combined MPPT 1 total of around 320V?

(3) Following on from (2) above), would it be better to try and aim for slightly smaller panels to enable 6 panels to be fitted (MPPT String 1), rather than 5, to achieve a higher voltage and get closer to the magic (?) number of 370V? My general thinking is that I’ve got a space limitation, hence to use fewer but larger panels (close to 600W?).  

(4) MPPT 2, String 1 (red block) is mocked-up to have 4 (possibly 5) panels all facing the same way, albeit on different roof panels. Can this work together as a single string?

(5) MPPT 2, string 2 (green line frame) faces the same direction as string 1 of the same MPPT, and presents space for 1 (and possibly 2) panels. Again the question is whether a single panel voltage would simply be added to the others on the other string that combine into the same MPPT, or whether anything less than 4 panels on a string wouldn’t work as the string doesn’t reach the minimum 150V. Unlike in point 1 above (albeit similar scenario), in this example the two strings’ panel(s) face into the same direction. Possible issue: distance of this panel to inverter location at top of pic?

(6) With two strings per MPPT, must one add the two amp ratings to ensure one remains under 22A per MPPT (ie 11amp per string)? Or worry less about that, provided that the volt max isn’t exceeded (i.e. all panels from the two strings added together must be under 450V or so?)

A bunch of questions 😚  but I hope they are clear enough with the detail provided and hopefully warrant short answers. The roof has its limitations, and I’d like to confine the number of panels if possible due to space constraints, and rather try to obtain larger panels (600W if they’re available, or as close to that); also to reduce need for mounting hardware. Certain spaces can clearly take 2 panels (even if large), but wouldn’t be able to take 3 even if considerably smaller. I’m also not willing to move or remove solar geysers and/or collectors. The whole roof aspect ratio is not optimal, and I’m aiming for more of an east-west kind of split. The east-ish facing roof is bathed in sun for many hours from early on in summer, obviously much less so in winter. In winter the whole system will obviously struggle a bit. 

Thanks very much in advance for inputs. 
 

Edited January 5, 20232 yr by Kalahari Cruiser

16 hours ago, Kalahari Cruiser said:

Hi all,
I’ve perused and learnt from a few forum threads on panel string sizing and am busy planning my own setup, to link in with a 8KW Sunsynk and 2x 5KW batteries. 

I did pose one or two string-related questions to SunSynk support while clarifying something else, but they deferred to solar installers for answers saying it required an in situ inspection. I suspect they might not have understood what I was trying to clarify. 

With the 8KW inverter having 2x MPPT inputs (each with two strings), I’m trying to plan panel sizing and config given the sub-optimal roof space and direction. It is what it is…

The image below has some possibilities that I’m considering. Note that panel sizes in the mockup are not to scale viz. the roof size and are all meant to be the same spec, and merely illustrate aspect and possible quantities given other limitations.

I’d appreciate brief answers on some specific questions - apologies for the number - which are basically all relating to string size and config:

Assumption: The inverters specs seem to be that each MPPT input can handle 22A which I assume means that two different strings feeding into each MPPT can handle panels with an A rating of less than 22A, subject to a V limit of 450V (or a bit less to be safe). 

 

(1) In terms of 2 strings per MPPT input, can each string's panels have different angles, provided that the panels within the same string face very broadly the same way? IOW, can one string of MPPT 1 face one way, and the second string of MPPT 2 face another (as in mockup, panels in yellow and pink frame), provided that both should receive sun together at more or less the same time hence providing volts to the SunSynk?

(2) Does each of the two strings within one MPPT have to individually meet the threshold of 150V start-up / minimum voltage to get the inverter going? In my mockup below, this is especially referencing MPPT 1 (String 2, pink line), where there is most definitely only space for 2 panels, i.e. a combined 90V-100V. String 1 (red line) on the same MPPT would always receive sun at the same time, and presumably (since there is space for 4-5 panels) would easily have say 250V. I think the optimal voltage is around 370V or so, so does this calculation add the 2 panels (say 90V) to the 5 panels (say 225V) to give a combined MPPT 1 total of around 320V?

(3) Following on from (2) above), would it be better to try and aim for slightly smaller panels to enable 6 panels to be fitted (MPPT String 1), rather than 5, to achieve a higher voltage and get closer to the magic (?) number of 370V? My general thinking is that I’ve got a space limitation, hence to use fewer but larger panels (close to 600W?).  

(4) MPPT 2, String 1 (red block) is mocked-up to have 4 (possibly 5) panels all facing the same way, albeit on different roof panels. Can this work together as a single string?

(5) MPPT 2, string 2 (green line frame) faces the same direction as string 1 of the same MPPT, and presents space for 1 (and possibly 2) panels. Again the question is whether a single panel voltage would simply be added to the others on the other string that combine into the same MPPT, or whether anything less than 4 panels on a string wouldn’t work as the string doesn’t reach the minimum 150V. Unlike in point 1 above (albeit similar scenario), in this example the two strings’ panel(s) face into the same direction. Possible issue: distance of this panel to inverter location at top of pic?

(6) With two strings per MPPT, must one add the two amp ratings to ensure one remains under 22A per MPPT (ie 11amp per string)? Or worry less about that, provided that the volt max isn’t exceeded (i.e. all panels from the two strings added together must be under 450V or so?)

A bunch of questions 😚  but I hope they are clear enough with the detail provided and hopefully warrant short answers. The roof has its limitations, and I’d like to confine the number of panels if possible due to space constraints, and rather try to obtain larger panels (600W if they’re available, or as close to that); also to reduce need for mounting hardware. Certain spaces can clearly take 2 panels (even if large), but wouldn’t be able to take 3 even if considerably smaller. I’m also not willing to move or remove solar geysers and/or collectors. The whole roof aspect ratio is not optimal, and I’m aiming for more of an east-west kind of split. The east-ish facing roof is bathed in sun for many hours from early on in summer, obviously much less so in winter. In winter the whole system will obviously struggle a bit. 

Thanks very much in advance for inputs. 
 

1. String 2 will pull string 1 down to their voltage and MPPT1 will not start. If in series they will provide better yields. 

2.  The optimal voltage AFAIK is a voltage range and not just at 370V.

The same applies  to MPPT2. 

Edited January 6, 20232 yr by Scorp007

Thanks Scorp007!

Yes, on speaking with a solar installer on Friday I was told the same (that "string 2" on MPPT 1). In fact it became clear that a 2-panel setup is a no-go here. I think that I was also told that both strings on the same MPPT should be facing the same way, so above scenario clearly contravenes that principle. 

Seems that it would/should then be whatever I can put on MPPT1 main roof (one string, 5 or 6 panels), and then MPPT as in picture, except to have what's labelled as string 2 1 or even 2 panels) also be part of MPPT2 / string 1 together with the rest, rather than on its own string. The aspect and angles are pretty much all the same. 

Was also dissuaded from thinking that trying to use 600W panels is much preferable (efficiency/output/cost) to say the 545W JA panels, given their real-world performance. Winter may be a bit of an issue, but in summer MPPT1 string is still bathed in full sun well after 1pm...

Where I have 1 panel on the small roof in above mockup I could probably fit 2x 545 space wise. panel placeholders are not to scale. 

On 2023/01/05 at 11:01 PM, Kalahari Cruiser said:

(6) With two strings per MPPT, must one add the two amp ratings to ensure one remains under 22A per MPPT (ie 11amp per string)? Or worry less about that, provided that the volt max isn’t exceeded (i.e. all panels from the two strings added together must be under 450V or so?)

It is most important that you don't exceed the Voltage spec to avoid catastrophic failure.

But don't ignore the current spec either. Aim for Imax under 11A per string, ie. panels that generate peak power at 11A or less, if you want to go parallel onto an MPPT. If you go higher like 13A or 17-18A, you will not get the specified peak performance from the panels, wasting money, unless you stick to a single series string.

Edited January 8, 20232 yr by GreenFields

Thanks @GreenFields

Yes it seems that I'll just be running one string on each MPPT in terms of available roof space and direction, and having a sufficient minimum number of panels per string. I think I'm not coming anywhere close to max VOC and am aware of that upper limit. 

Nothing wrong with adding 5S say East and 5 S West and connect them into the same MPPT provided the Imp is 11A or below which means panels of about 455W as was indicated already. You just never want to put series of panels where some are East and some West. The same applies with North and South. 

2 hours ago, Kalahari Cruiser said:

 I think that I was also told that both strings on the same MPPT should be facing the same way, so above scenario clearly contravenes that principle. 

They can face different directions.

The two inputs in each MPPT are connected in parallel meaning the one string will not be impacted by a voltage drop on the other string.

It is common for an east facing string and a west facing string to be combined in one MPPT and with, by example a North facing string, on MPPT2.

19 minutes ago, I84RiS said:

The two inputs in each MPPT are connected in parallel meaning the one string will not be impacted by a voltage drop on the other string.

Thanks both!

That’s very interesting, and makes a lot of sense to me, else why would a two-string MPPT be of much value if all panels had to be the same aspect. However, it’s completely contrary to what solar installer company told me Friday, during in situ inspection. 

In any case, it’s probably a bit of a moot point for my roof, since where I thought of adding two extra panels (opening post, MPPT1, string 2) I would only be able to fit 2 panels, meaning (unless I’m mistaken) that string would only deliver max 90V or so. Installer told me the other string on that MPPT would be brought down to the voltage of that second string, being on the same MPPT. 
 

I guess that much is probably true, since it would be on a separate string, and not be seen as a series calculation for MPPT voltage sizing, right? In other words, even if string faces a different way and would be lit up by sun at the same time as string 1 on that MPPT, the string voltage from 2 panels would be too low to be an effective player. Perhaps my understanding of this is wrong. 

Edited January 8, 20232 yr by Kalahari Cruiser
Typo

Yes those 2 panels at 90V will bring the 4 or more panels down as I alerted in my 1st reply. 

Panels that get light will be close to the same voltage that's why one can parallel east and west. It is the irradiation on each set that will drive the current each set will produce. 

Ok, yes thanks again. So hypothetically, if that string 2 (MPPT 1) with space for 2 panels had space for 4-5 panels, despite being at 90 degrees to string 1 on same MPPT, it would work fine. The limitation here is there’s only physical space for 2 panels, given that it’s a small extension / roof. Second mock-up is probably a more feasible solution. 

5 minutes ago, Kalahari Cruiser said:

Ok, yes thanks again. So hypothetically, if that string 2 (MPPT 1) with space for 2 panels had space for 4-5 panels, despite being at 90 degrees to string 1 on same MPPT, it would work fine. 

Yes they would then work fine if selected panels Vmp is over about 165V to start. 

Apologies for the thread revive, but I have a very similar scenario and questions for a new install like @Kalahari Cruiser.  

Spec of install:

8KW Sunsynk and 2x 5KW SunSynk batteries. (Or 2x 5KW Synsynk)

12x 550w panels (6x North; 3x East; 3x West)

I have heard conflicting info from installers, and have read this thread and others to try and understand this better.  My current installer have quoted 2x 5KW SynSynk inverters, with the reasoning that 2 inverters are needed to accommodate the 3 pv strings.  From what I have read here, it shouldn't be necessary, and I can rather use a 8KW SS inverter, and combine the east and west strings into one mppt, and north facing panels into the other mppt.  I have reached out to other installers, which have commented that the 8KW inverter with 3 string is the way to go, and installing 2x 5KW is just a way for the installer to push up the price.  

Do I understand this correct?

Thanks in advance for any insight.

For anyone to assist, we need the specs of your panels.

Take a pic of the data label on the panel and upload here.

19 hours ago, VicB said:

Apologies for the thread revive, but I have a very similar scenario and questions for a new install like @Kalahari Cruiser.  

Spec of install:

8KW Sunsynk and 2x 5KW SunSynk batteries. (Or 2x 5KW Synsynk)

12x 550w panels (6x North; 3x East; 3x West)

I have heard conflicting info from installers, and have read this thread and others to try and understand this better.  My current installer have quoted 2x 5KW SynSynk inverters, with the reasoning that 2 inverters are needed to accommodate the 3 pv strings.  From what I have read here, it shouldn't be necessary, and I can rather use a 8KW SS inverter, and combine the east and west strings into one mppt, and north facing panels into the other mppt.  I have reached out to other installers, which have commented that the 8KW inverter with 3 string is the way to go, and installing 2x 5KW is just a way for the installer to push up the price.  

Do I understand this correct?

Thanks in advance for any insight.

My 1st question to the installer would be how is he going to connect the 3 strings in the 2x5kw inverter scenario. 

A single 5kw or 8kw inverter you can used for 12 panels if 2 strings are used made up of 3N in series with 3E in a string and 3N and 3W in the 2nd string. 

This will create some reduced output in the morning as well as afternoon. During these periods the north string should reduce the output from the east and west panels. 

The same should happen where the east and west can reduce the output from the north panels during peak period of the day. 

This is how I see it. 

Edited June 24, 20231 yr by Scorp007