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Sizing a string for SunSynk 5KW Inverter


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I need some help sizing a correct string for a SunSynk 5KW inverter. The inverter specs are as following:

PV String Input Data
Max. DC Input Power 6500W
PV Input Voltage 370V (100V~500V)
MPPT Range 125~425V
Full Load DC Voltage Range 240~425V
Start-up Voltage 150V
PV Input Current 11A+11A
No. of MPPT Trackers 2
No. of Strings Per MPPT Tracker 1+1
 

I would like to size a string for one MPPT Tracker and possibly half of the input power for now. Later I'll add more panels for another sting on the second MPPT.

The panels I have in mind have the following specs:

Electrical Characteristics STC            
Maximum Power (Pmax)                    250 W
Maximum Power Current (lmp)          8.31 A
Maximum Power Voltage (Vmp)        30.1 V
Short Circuit Current (lsc)                  8.83 A
Open Circuit Voltage (Voc)                37.4 V

Electrical Characteristics NOCT
Maximum Power (Pmax)                    185 W
Maximum Power Current (lmp)          6.57 A
Maximum Power Voltage (Vmp)        28.2 V
Short Circuit Current (lsc)                  7.12 A
Open Circuit Voltage (Voc)                35.0 V

 

First question would be - Do I look at the specs at STC or NOCT?

Another question - What does it mean when they say "Full Load DC Voltage Range 240~425V"? Is the MPPT going to stop working above 425V?

And thirdly - What do they mean by "No. of Strings Per MPPT Tracker 1+1"? Can I put 2 strings per MPPT and use a combiner box or only 1 string per MPPT, as there is only 2 PV inputs on the inverter?

If I want to be as close as possible to half the inverter DC power 6500W/2 = 3250W

Let's say the panels are old already lost 10% of the max power, so they should be 225W now. 

3250W/225W = 14.4 panels. Let's say 14 panels in one string per this calculation.

Or, as per NOCT - 185W - 10% = 166.5W

3250W/166.5 = 19.5 panels. Let's say 19 panels in one string per this calculation.

The inverter say - Full Load DC Voltage Range 240~425V

425V/37.4V = 11.4 panels. Let's say 11 panels in one string per this calculation.

Or if I look at the NOCT Voc 35V:

425V/35V = 12.1 panels. Let's say 12 panels in one string per this calculation.

 

Which of the above calculation and string size would be best given the above specification?

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Winter is when i run my solar at max, as the days are shorter and solar output is less and more power is required for heaters. I would use then take say 3250w / 185w = 17.5 panels per string. But 17 panels x 37.4 volts = 635 volts, which would blow your inverters mppt. I would recomend a panel that has an imp @ stc much closer to 11, to be able to get full power from the inverter during winter. I like the JA solar JAM72S20 panels.

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On 2021/10/15 at 3:56 PM, PowerUser said:

I need some help sizing a correct string for a SunSynk 5KW inverter. The inverter specs are as following:

PV String Input Data
Max. DC Input Power 6500W
PV Input Voltage 370V (100V~500V)
MPPT Range 125~425V
Full Load DC Voltage Range 240~425V
Start-up Voltage 150V
PV Input Current 11A+11A
No. of MPPT Trackers 2
No. of Strings Per MPPT Tracker 1+1
 

I would like to size a string for one MPPT Tracker and possibly half of the input power for now. Later I'll add more panels for another sting on the second MPPT.

The panels I have in mind have the following specs:

Electrical Characteristics STC            
Maximum Power (Pmax)                    250 W
Maximum Power Current (lmp)          8.31 A
Maximum Power Voltage (Vmp)        30.1 V
Short Circuit Current (lsc)                  8.83 A
Open Circuit Voltage (Voc)                37.4 V

Electrical Characteristics NOCT
Maximum Power (Pmax)                    185 W
Maximum Power Current (lmp)          6.57 A
Maximum Power Voltage (Vmp)        28.2 V
Short Circuit Current (lsc)                  7.12 A
Open Circuit Voltage (Voc)                35.0 V

 

First question would be - Do I look at the specs at STC or NOCT?

Another question - What does it mean when they say "Full Load DC Voltage Range 240~425V"?Is the MPPT going to stop working above 425V?

And thirdly - What do they mean by "No. of Strings Per MPPT Tracker 1+1"? Can I put 2 strings per MPPT and use a combiner box or only 1 string per MPPT, as there is only 2 PV inputs on the inverter?

If I want to be as close as possible to half the inverter DC power 6500W/2 = 3250W

Let's say the panels are old already lost 10% of the max power, so they should be 225W now. 

3250W/225W = 14.4 panels. Let's say 14 panels in one string per this calculation.

Or, as per NOCT - 185W - 10% = 166.5W

3250W/166.5 = 19.5 panels. Let's say 19 panels in one string per this calculation.

The inverter say - Full Load DC Voltage Range 240~425V

425V/37.4V = 11.4 panels. Let's say 11 panels in one string per this calculation.

Or if I look at the NOCT Voc 35V:

425V/35V = 12.1 panels. Let's say 12 panels in one string per this calculation.

 

Which of the above calculation and string size would be best given the above specification?

1. Do I look at the specs at STC or NOCT?: Both, It depends on what you want to measure. This is a good guide: https://unboundsolar.com/blog/string-sizing-guide

2. Is the MPPT going to stop working above 425V? Keith Gough answered this question here. I have never been over that, so I cannot tell you from experience. It is clear that anything over 500V (Keith quotes 480V) will result in expensive repairs (blowing the DC-->DC converter), and general consensus is to keep things below 450V. Leave some headroom for voltage rising when there is no load (use VoC, not VmP), and cloud edge effect. What we do know is that the MPPT does clip the amps, Sunsynks with latest firmware are limit it to 13A per MPPT. Voltage is a little more difficult to control, so make very sure that you get your calculations right. On the 5.5K Sunsynk you basically have 2.75kW per MPPT to play with, that's quite generous.

3. What do they mean by "No. of Strings Per MPPT Tracker 1+1"? Can I put 2 strings per MPPT and use a combiner box or only 1 string per MPPT, as there is only 2 PV inputs on the inverter?

😯 Is that a genuine Sunsynk 5K or a knock-off? As per Sunsynk installer manual, there are 2 inputs for each MPPT:

image.png.bed13ef33e66cd755ee8c00f147a2674.png

image.png.07f9cbc2b766ec89627a466d2391a52c.png

The guide should answer all of your other questions, take a look, and draw up a spreadsheet with your panel specs. Bear in mind that STC and NOCT are calculated using pristine conditions in a lab. You already alluded to the fact that the panels are 1 year old, that's using good judgement, look at the degradation graph on the specs and use it, also use your own judgement when it comes to azimuth and pitch / tilt. Bear in mind that STC and NOCT are calculated using pristine conditions in a lab.

Wishing you the all the best with the install. Post some pics!

 

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On the panel Volts, use the highest one of STC or NOCT and base the calcs on VoC. Most panels should have a temp graph, look that up. The colder the temps (winter early mornings) the higher the VoC. This means that the VoC will rise or get more the lower the temps. The graphs are normally based on standard temps, like 25Celsius. Some panels will show VoC for colder temps on the graphs. Use that to see what the VoC will be at say -5Celsiuc for Johannesburg and figure out how many panels will get you at a reasonable voltage, and not to close to max, say max volts minus 5 or 8% or whatever you like for a safety margin.
 

The inverter MPPT cannot withstand overvoltages, so stay well clear of max volts, it will smoke beyond max volts.

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Thank you @De0n19, @YellowTapemeasure and @Sarel for the replies!

16 hours ago, YellowTapemeasure said:

3. What do they mean by "No. of Strings Per MPPT Tracker 1+1"? Can I put 2 strings per MPPT and use a combiner box or only 1 string per MPPT, as there is only 2 PV inputs on the inverter?

😯 Is that a genuine Sunsynk 5K or a knock-off? As per Sunsynk installer manual, there are 2 inputs for each MPPT:

image.png.bed13ef33e66cd755ee8c00f147a2674.png

 

 

 

 

@YellowTapemeasure, of course mine is a genuine SunSunk. Never heard of knock-off SunSynks 🙃

The diagram you posted above is for 8KW version. Mine is 5KW with 2 PV inputs only. Here is the correct diagram for mine:

sun1.PNG.2b6319a6b552c8533b5e3ffbf5c32fe8.PNG

 

Hence my question, what do they mean in the datasheet by saying "No. of Strings Per MPPT Tracker 1+1"?

I have 2 MPPTs and only one input for each.

 

I will redo the calculations by using the guide from your links and Sarel's advice for temperature correction and post the new results here.

 

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There is an Xcell spreadsheet from Victron where you can add your own panel data and simulate the panel voltages with temp. You welcome to ignore the Victron bits, just select something closeish to what your specs are and simply use the sheet to model the panels, temps etc….

Panel calc here

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@PowerUser, correct, the sunsynk 5 kW has two mppt’s with ONE string each with a maximum current rating of 13 amps per string with updated firmware.

 Are you sure that you want to use 250 watt panels, that would use  a lot of roof mounting structure, the norm nowadays is to use 400 plus watt panels, even the new JaSolar 540 watt panels

 

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1 hour ago, PowerUser said:

Thank you @De0n19, @YellowTapemeasure and @Sarel for the replies!

 

@YellowTapemeasure, of course mine is a genuine SunSunk. Never heard of knock-off SunSynks 🙃

The diagram you posted above is for 8KW version. Mine is 5KW with 2 PV inputs only. Here is the correct diagram for mine:

sun1.PNG.2b6319a6b552c8533b5e3ffbf5c32fe8.PNG

 

Hence my question, what do they mean in the datasheet by saying "No. of Strings Per MPPT Tracker 1+1"?

I have 2 MPPTs and only one input for each.

 

I will redo the calculations by using the guide from your links and Sarel's advice for temperature correction and post the new results here.

 

Just whipped the covers of my own Sunsynk to check. The ones that come with the MC4 connectors are the knockoffs 😂🤣

Just joking, you are correct, my apologies. Looks like per MPPT, 1+1=1 

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8 minutes ago, Tariq said:

@PowerUser, correct, the sunsynk 5 kW has two mppt’s with ONE string each with a maximum current rating of 13 amps per string with updated firmware.

 Are you sure that you want to use 250 watt panels, that would use  a lot of roof mounting structure, the norm nowadays is to use 400 plus watt panels, even the new JaSolar 540 watt panels

 

I suspect that @PowerUser got them in a good deal as used...

 

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@YellowTapemeasure I still haven't gotten anything yet. I'm just looking at optimum string sizes and learning how to calculate them properly, so I can compare various options. I created an Excel spreadsheet with the formulas from your link and now I can easily compare options. Here is for example 2 options for JA Solar from the PowerForum store:

calc2.PNG.c0c0953121cd3b8810ceed481ec2f2d9.PNG

For the calculation, I used minimum possible temperature for Randburg -5deg C. The minimum, I could find on record for the past 10 years was -4 deg C. However, that is still very conservative value, as it's probably measured at 4AM or 5AM in winter, when the sun is not shining. We should be looking at minimum temp when the sun is out but doubt, I will find a record like that.

Obviously, the above is only based on the electrical parameters, assuming roof space is not an issue.

There is no voltage drop in the cables accounted in the calculation. Not sure if that will be of any significance for less than 20m cable lengths.

Which option do you guys say would be better? I think, ignoring the cost, the 540W are closer to the half DC PV input power of 6500W/2 = 3250. I'm thinking that because of the Maximum theoretical power by max string at NOCT.

 

Edited by PowerUser
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As for the first panels, I posted the specs previously, someone is selling those in the classified section and I'm trying to figure out, if they are going to be a good match for the inverter. However, despite the lower cost, I think, a string of those will be way too undersized for the SunSynk 5kW.

What is everyone else's opinion?

calc3.PNG.24b877bb2e990909bc15804c73102191.PNG

Edited by PowerUser
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8 hours ago, PowerUser said:

For the calculation, I used minimum possible temperature for Randburg -5deg C. The minimum, I could find on record for the past 10 years was -4 deg C. However, that is still very conservative value, as it's probably measured at 4AM or 5AM in winter, when the

Normally, the coldest temps are after Sunrise, it takes a while for Mr Sun to warm things a bit. At my place, using a calibrated weather station, I have seen coldest temps about 1 hour after Sunrise.

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16 hours ago, PowerUser said:

As for the first panels, I posted the specs previously, someone is selling those in the classified section and I'm trying to figure out, if they are going to be a good match for the inverter. However, despite the lower cost, I think, a string of those will be way too undersized for the SunSynk 5kW.

What is everyone else's opinion?

calc3.PNG.24b877bb2e990909bc15804c73102191.PNG

Pros: Clearly the 540W panels, have a higher amperage than all the rest, are the winners in terms of production, and the current is nowhere near the clipping threshold of the 5.5K Sunsynk's MPPT (13A). Their footprint is also smaller, so less exposure to the risk of shading, trees tend to grow substantially in 20 years, and who knows whether a neighbors may decide to erect a building in the same time. There is also some saving in terms of mounting hardware.

Cons: Looking at the difference between the 380 and 450W panels, is it an extra 400w really worth R6000? This is a question that only you can answer, if you have the funds available, them maybe, if you have other priorities, or need to borrow the money (and add interest), then maybe not.  

 

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Instead of just using a spreadsheet it helps to understand how/why the calc is required/done.

Here is a write up for the OP to figure it all out...Can't remember where I got it from and cant take credit for it.

As temp decreases, panel Voltage(V) increases. We want to calculate the maximum Voc for a panel at the minimum temp we expect in the region. So for JHB we use 0’C, in Bloemfontein we use -10C or whatever you chose...

To calculate string length (series) use the total Voc (Sum of voltages, because they are in series remember, (+) including adjustment for cold temp) from panel data sheet and then do as follows;

 Record-low temperature: -10ºC (From your region, town)
Temperature coefficient of (VOC): – (0.30) %/ºC (From data sheet *)
Module open circuit voltage (VOC): 39.4 V *
Inverter maximum input voltage: 600V (From inverter, MPPT data sheet)

The STC temperature is 25ºC. This temperature needs to be deducted from the array location’s record-low temperature of -10 degrees as follows:

25 – (-10) = 35º difference.

Multiply the 35º difference by the temperature coefficient of VOC (I’ve used the positive value for an easier calculation, though you get the same result) then multiply by the module’s VOC:

35 x 0.0030 = 0.105
0.105 x 39.4V = 4.137V (So the Voc will increase by 4.13 V if the temp goes down to -10C)

This is how many volts each pv module will increase due to record-low temperatures.

Add the voltage increase to the Module VOC.

Then divide the inverter maximum input voltage by that number. This will give you the maximum number of modules that can be wired in a series string per that inverter and specific location.

4.137 V (from calculation due to low max low temperature) + 39.4V (from datasheet) = 43.537 Vmax (At -10C) Increase in voltage due to temp drop per panel.

Normal voltage at 25C
600V / 43.537 = 13.7 Panels (round down to a whole number)

The maximum number of pv modules in this series string is 13.

A series string of 14 could potentially produce more than 600V during record-low temperatures.

When string in parallel, add Current, V stays the same, when in series, add Voltage, I stays the same.

 

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10 minutes ago, FixAMess said:

Instead of just using a spreadsheet it helps to understand how/why the calc is required/done.

Here is a write up for the OP to figure it all out...Can't remember where I got it from and cant take credit for it.

As temp decreases, panel Voltage(V) increases. We want to calculate the maximum Voc for a panel at the minimum temp we expect in the region. So for JHB we use 0’C, in Bloemfontein we use -10C or whatever you chose...

To calculate string length (series) use the total Voc (Sum of voltages, because they are in series remember, (+) including adjustment for cold temp) from panel data sheet and then do as follows;

 Record-low temperature: -10ºC (From your region, town)
Temperature coefficient of (VOC): – (0.30) %/ºC (From data sheet *)
Module open circuit voltage (VOC): 39.4 V *
Inverter maximum input voltage: 600V (From inverter, MPPT data sheet)

The STC temperature is 25ºC. This temperature needs to be deducted from the array location’s record-low temperature of -10 degrees as follows:

25 – (-10) = 35º difference.

Multiply the 35º difference by the temperature coefficient of VOC (I’ve used the positive value for an easier calculation, though you get the same result) then multiply by the module’s VOC:

35 x 0.0030 = 0.105
0.105 x 39.4V = 4.137V (So the Voc will increase by 4.13 V if the temp goes down to -10C)

This is how many volts each pv module will increase due to record-low temperatures.

Add the voltage increase to the Module VOC.

Then divide the inverter maximum input voltage by that number. This will give you the maximum number of modules that can be wired in a series string per that inverter and specific location.

4.137 V (from calculation due to low max low temperature) + 39.4V (from datasheet) = 43.537 Vmax (At -10C) Increase in voltage due to temp drop per panel.

Normal voltage at 25C
600V / 43.537 = 13.7 Panels (round down to a whole number)

The maximum number of pv modules in this series string is 13.

A series string of 14 could potentially produce more than 600V during record-low temperatures.

When string in parallel, add Current, V stays the same, when in series, add Voltage, I stays the same.

 

Hello @FixAMess, your explanation is exactly what from the link, @YellowTapemeasure provided earlier in the thread. I have gone through those explanations and created the spreadsheet with the calculations myself based on exactly those explanations. Your post is just going backward in a circle.

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  • 3 weeks later...

Which one of these 2 options - JASolar 540W or CanadianSolar 595W panels would fit better one MPPT of the 5kW SunSynk better?

In the calculations guide which I used - PV current is not taken into consideration. The JASolar current is Imp = 12.97A, The CanadianSolar current is Imp = 17.15A. So looks like the CanadianSolar are producing more power by increasing the current and not the voltage.

According to what the forum users are saying, the SunSynk with the latest firmware should have PV Input Current of 18A per MPPT. The datasheet lists 11A only. I'm not sure, if because of that the CanadianSolar will be a better option.

But on the other hand a string of CanadianSolar can produce Maximum theoretical power by max string NOCT = 4450W. The JASolar can produce Maximum theoretical power by max string NOCT = 3264W. The Max. DC Input Power for the inverter is 6500W. So, if I add another string later, with JASolar, the 2 MPPT inputs may be are going to be balanced better.

Let me know of your opinions please!

 

pv1.PNG.1c027b6366496c2c0cc7dd1ea9f1f81a.PNG

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11 hours ago, PowerUser said:

Thanks for the correction @Tariq. I'm wrong then. Not sure why I remembered 18A instead of 13A.

In this case - yes, I will be wasting power with the specified model CanadianSolar because of the AMPs clipping. Not a worthy option.

18A was the limit of the 8K Sunsynk's MPPT, before its upgrade to 22A.

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6 minutes ago, Gerry R said:

I have 13x535 watt panels connected to my 5kW Sunsynk. 7 on 1 string and 6 on the other. 7 on main roof of house and 6 on a garage roof. Both north facing. Maximum current that I have seen drawn has been 12,5 amps.

Capture.JPG

Thank you @Gerry R ! What is the exact brand/model of your panels?

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