I've been trying to wrap my head around the system I am looking to install, and I'd appreciate some input if you have some to spare please:)

 

For a guide, I've been following this site: https://solardesignguide.com/calculating-string-size/

It feels like a fairly good place to start in getting my calculations right, but there's something fairly trivial that I'm not understanding.

 

By way of example, let's assume a Solis S6 (Tech Specs: https://drive.google.com/file/d/19Ay6UZRCvoYI2LsUeAHu-b2gQaiDDdjx/view)

and Canadian Solar HiKu6 Mono PERC 550W (Tech Specs: https://static.csisolar.com/wp-content/uploads/sites/9/2023/09/26103742/CS-Datasheet-HiKu6_CS6W-MS_v2.8_EN-2278mm-1.pdf)

The inverter's Max Input Voltage is 600V with the MPPT range between 90 and 520. In my area, the coldest day I can find is about 8 degrees C. So with a temperature coefficient  of 0.26%/degC, the Voc in theory can jump from 49.6 to 51.8.

That gives me a theoretical max of 11 panels (11.5 panels rounded down). 11 panels gets me up to a max of 6kW.

The inverter spec has a Recommended max. PV power of 9.6kW.

The inverter has 2 strings. I guess my questions are

1. Are my calculations correct?
2. Is Max PV Input voltage actually 600V on each string?
3. Would this mean I have a second string that I can add up to 3.4kW?
4. Should I be looking at 2 equally sized strings?

Should I be looking at a a different spec PV panel to achieve the best matching where I'm maxxing out production within the right limits of the inverter?

Would be happy to move up and down the range to get the optimal PV panels sizes to max out the inverter (the panels will never be in the perfect orientation to max it out anyway, so I want to ensure that I don't set it up too far under what I can get).

Also going to run similar calcs on the sunsynk 5kW, but will post a separate question on inverter comparisons (software etc). Right now my brain says 2 x 6kW inverter with 16A current > 2 x 5kW inverter with 13A current.

I know this is a mouthful to read, so any insight here would be amazing, thank you.

 

I think you should look at two strings that are the same, unless you have constraints with space, shape or direction of your roof.

This is just to keep closer to the rated Voltage of 330V, and away from extremes. I mean, if there's no reason to approach the limits, then just don't.

In this case with these components you could consider 8 panels per MPPT, which would give you 8.8kW of panels in total on a 6kW inverter, and a more middle of the range 414Voc, dropping to 8*41.7=333Vmp during operation, which is about bang-on in this inverter's rated Voltage.

If you've got East and West-facing sections of roof, you could consider 9 per MPPT, but already with 8 you'll be doing well with a fairly decent system. Bear in mind that anytime above 6kW the inverter should be clipping AC output anyway.

Otherwise play around with other panel ratings, maybe a 400W or 480W panel to get precisely on the inverter's max rating of 9600W, even if the Voltage is a bit further away from the 330V. In that case you may find that the cost per Watt of the smaller panels, a higher number of panels, plus installation costs and sundries, reduces the attractiveness of the extra Watts. But maybe you won't know until you run the numbers and get quotes.

 

 

1 hour ago, display_Name said:

I've been trying to wrap my head around the system I am looking to install, and I'd appreciate some input if you have some to spare please:)

 

For a guide, I've been following this site: https://solardesignguide.com/calculating-string-size/

It feels like a fairly good place to start in getting my calculations right, but there's something fairly trivial that I'm not understanding.

 

By way of example, let's assume a Solis S6 (Tech Specs: https://drive.google.com/file/d/19Ay6UZRCvoYI2LsUeAHu-b2gQaiDDdjx/view)

and Canadian Solar HiKu6 Mono PERC 550W (Tech Specs: https://static.csisolar.com/wp-content/uploads/sites/9/2023/09/26103742/CS-Datasheet-HiKu6_CS6W-MS_v2.8_EN-2278mm-1.pdf)

The inverter's Max Input Voltage is 600V with the MPPT range between 90 and 520. In my area, the coldest day I can find is about 8 degrees C. So with a temperature coefficient  of 0.26%/degC, the Voc in theory can jump from 49.6 to 51.8.

That gives me a theoretical max of 11 panels (11.5 panels rounded down). 11 panels gets me up to a max of 6kW.

The inverter spec has a Recommended max. PV power of 9.6kW.

The inverter has 2 strings. I guess my questions are

1. Are my calculations correct?
2. Is Max PV Input voltage actually 600V on each string?
3. Would this mean I have a second string that I can add up to 3.4kW?
4. Should I be looking at 2 equally sized strings?

Should I be looking at a a different spec PV panel to achieve the best matching where I'm maxxing out production within the right limits of the inverter?

Would be happy to move up and down the range to get the optimal PV panels sizes to max out the inverter (the panels will never be in the perfect orientation to max it out anyway, so I want to ensure that I don't set it up too far under what I can get).

Also going to run similar calcs on the sunsynk 5kW, but will post a separate question on inverter comparisons (software etc). Right now my brain says 2 x 6kW inverter with 16A current > 2 x 5kW inverter with 13A current.

I know this is a mouthful to read, so any insight here would be amazing, thank you.

 

1. It looks right

2. Each string. 

3. Yes

4. They need not be the same. 

17 minutes ago, Scorp007 said:

1. It looks right

2. Each string. 

3. Yes

4. They need not be the same. 

Brilliant, thank you.

58 minutes ago, GreenFields said:

I think you should look at two strings that are the same, unless you have constraints with space, shape or direction of your roof.

This is just to keep closer to the rated Voltage of 330V, and away from extremes. I mean, if there's no reason to approach the limits, then just don't.

In this case with these components you could consider 8 panels per MPPT, which would give you 8.8kW of panels in total on a 6kW inverter, and a more middle of the range 414Voc, dropping to 8*41.7=333Vmp during operation, which is about bang-on in this inverter's rated Voltage.

If you've got East and West-facing sections of roof, you could consider 9 per MPPT, but already with 8 you'll be doing well with a fairly decent system. Bear in mind that anytime above 6kW the inverter should be clipping AC output anyway.

Otherwise play around with other panel ratings, maybe a 400W or 480W panel to get precisely on the inverter's max rating of 9600W, even if the Voltage is a bit further away from the 330V. In that case you may find that the cost per Watt of the smaller panels, a higher number of panels, plus installation costs and sundries, reduces the attractiveness of the extra Watts. But maybe you won't know until you run the numbers and get quotes.

 

 

Thank you. I'm busy plotting this all out in excel at the moment... and I'm also looking at smaller panels as well (460W range). Some of it will come down to roof space and how much space I might need to leave near the edges. Was also thinking of dropping string size to keep voltage within MPPT if I'm going to be limited in any case by having one smaller string. I'll revert here with my thinking if you don't mind having another look. Thank you.

3 hours ago, display_Name said:

The inverter has 2 strings.

The inverter has 4 strings, 2 strings/mppt. Mppt1 2 x strings and Mppt2 2 x strings. The max input power/Mppt is 4800 watts and should not be exceeded. Each string/ Mppt should be the same.

23 minutes ago, TaliaB said:

The inverter has 4 strings, 2 strings/mppt. Mppt1 2 x strings and Mppt2 2 x strings. The max input power/Mppt is 4800 watts and should not be exceeded. Each string/ Mppt should be the same.

Ah, I missed the 2/2. Thank you. What is the giveaway that it's max power/mppt is 4800? Is it just that the max power is halved because it's 2 x mppt? Then do you mean that each string on the same mppt must be the same, but the second mppt can have different string size?

Related question... Is it better to use a single string per MPPT to get a higher minimum voltage? My roof is a peaked roof and I will have panels on 2 sides only... so no need to have 4 different orientations. So orientation aside... my assumption is 1 string used per MPPT?

Edited February 7, 20242 yr by display_Name

31 minutes ago, display_Name said:

Ah, I missed the 2/2. Thank you. What is the giveaway that it's max power/mppt is 4800? Is it just that the max power is halved because it's 2 x mppt? Then do you mean that each string on the same mppt must be the same, but the second mppt can have different string size?

Related question... Is it better to use a single string per MPPT to get a higher minimum voltage? My roof is a peaked roof and I will have panels on 2 sides only... so no need to have 4 different orientations. So orientation aside... my assumption is 1 string used per MPPT?

Yes higher voltage more panels/ string. ( But stay well away of 600v 520v max)also don't exceed the total wattage of 9600w it is there for a reason. 

 

 

31 minutes ago, display_Name said:

but the second mppt can have different string size?

Yes the 2 Mppt's are seperate from each other. For example you can use 1Mppt for East orientation and the other for West orientation with different amount of panels.

Edited February 7, 20242 yr by TaliaB

26 minutes ago, TaliaB said:

Yes higher voltage more panels/ string. ( But stay well away of 600v 520v max)

Ok yeah, as per GreenFields... perhaps 8 panels (if they are 550W) per MPPT would be a sweet spot... and that would only be a Voc of414V and max power of 4400W.

15 minutes ago, display_Name said:

Ok yeah, as per GreenFields... perhaps 8 panels (if they are 550W) per MPPT would be a sweet spot... and that would only be a Voc of414V and max power of 4400W.

I would not want to disagree with the point of keeping both MPPTs at the same power levels but feel I have to add many inverters can have different power on each MPPT. 

Running a east - west string to each MPPT will have a different power to each MPPT except at midday. 

This can be inverter specific. 

If it was important to keep the Max wattage per MPPT to below 4800W (as TaliaB said above), then on my roof it would likely make sense to keep them both at max panels, and that would mean balancing them at 8 panels a piece. If I was able to go over 4800W, then I'd probably have one MPPT with more panels, but at this rate, I want to max out each one.

 

Next (last? probably not last tbh) question on strings etc. The MPPT supports 2 strings. The strings need to be the same size if I use 2 strings. I'm probably going with one string per MPPT, and putting all 8 panels on the one string to max out voltage for earlier morning kick in, and for winter etc. However... let's say I want to utilise another portion of my roof because maybe I can better utlise some sun for a portion of the day... can I face 2 strings on a single MPPT in different directions? Would that not have the same effect as making the two strings on the one MPPT a different size?

 

And to your point about East / West... I do have a bit of an East / West split... with the one side getting more morning sun, and the other more afternoon. Was wondering if I could put one string from each MPPT on each side of the roof, or whether I put one MPPT (single or both strings) on each side?

Edited February 7, 20242 yr by display_Name

9 minutes ago, display_Name said:

If it was important to keep the Max wattage per MPPT to below 4800W (as TaliaB said above), then on my roof it would likely make sense to keep them both at max panels, and that would mean balancing them at 8 panels a piece. If I was able to go over 4800W, then I'd probably have one MPPT with more panels, but at this rate, I want to max out each one.

 

Next (last? probably not last tbh) question on strings etc. The MPPT supports 2 strings. The strings need to be the same size if I use 2 strings. I'm probably going with one string per MPPT, and putting all 8 panels on the one string to max out voltage for earlier morning kick in, and for winter etc. However... let's say I want to utilise another portion of my roof because maybe I can better utlise some sun for a portion of the day... can I face 2 strings on a single MPPT in different directions? Would that not have the same effect as making the two strings on the one MPPT a different size?

 

And to your point about East / West... I do have a bit of an East / West split... with the one side getting more morning sun, and the other more afternoon. Was wondering if I could put one string from each MPPT on each side of the roof, or whether I put one MPPT (single or both strings) on each side?

This might provide some answers. Currently you seem to have a few active topics. 

1 hour ago, Scorp007 said:

This might provide some answers. Currently you seem to have a few active topics. 

Thanks. I still need lots of reading. So many questions in my head right now regarding everything.