String Theory & Amp Question for 5Kw panel & grid tie install

KenInNC

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Mar 26, 2020
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Hello! First timer here. I am trying to install my panels this April and I am ordering the PV cable and MC 4 connectors and I wanted to hear if I have my math correct or wrong please.

I am using a Rensola RePlus 8000TL Plus Grid Tie String Inverter and SunPower Performance Series SPR-P17-340-COM panels. I can use either 14 or 15 panels at this portion of the first install and I plan to add the others to bring the power as close to 8000w as possible so I have the choice of using 3 sets of 5 panels in series or 2 sets of 7 panels in series joined as the 1st string to the Inverter (as I believe it can also allow me to add a 2nd final string later (additional 3Kw of panels)).

I will attach the .pdf's of the panels and inverter but to be fast I did my best to enter the direct information for the specific models of each item. I hope this helps.

Quick history. I have my install permit from the county, the 8000w Inverter, 10Kw worth of 340w panels, mounting hardware, Combiner box, emergency disconnects and MC4 M&F connectors. I am getting ready to order Marine Grade 10AWG sheathed two wire (red and black), 2 to 1 and 3 to 1 MC4 combiners connectors, MC4 wrenches. I am debating on a thru the roof to the attic roof openning versus a long run of direct exposed cable (in proper outdoor gray plastic EMT pipe). I am open to any suggestions. I will try to post a rough drawing of my roof layout and limited panel placement and hopefully number the panels to make anyone's explanation easier to do. Thank you for reading this far as I am a long winded type of guy who tries to provide the most information the first time around.

My concerns are:
1.
I am trying to order some 2:1 and 3:1 MC4 pair branch connectors before the delivery starts getting worse with this Covid 19 (I want to get the order right the first time). I will be running 10AWG Marine Grade wire for all connections.

2. I don't understand the start voltage for the inverter. Does this mean that the rated start voltage of 360Vdc needs my string from 14-15 42.5Vdc panels to add up to at least the 360Vdc or more but not more than 600Vdc? I was thinking of doing three sets of five panels which has a combined series and final parallel voltage total of 212.5Vdc at 24 A. So I guess this does not work?

3. Should I focus on getting a series of panels to add up to at least 360Vdc and is the higher voltage the better up to 600Vdc? This may limit what I can put to the inverter as I was told that two strings going into the inverter should match voltage as close as possible between the two strings. Is this correct too?

4. With 15 panels, I can combine 8 for a series Vdc total of 340Vdc and combine the other 7 for a series Vdc total of 297.5 (all under optimum conditions of course) but this sounds like the inverter will not even start with this Vdc power incoming plus the two sets of incoming would be what ... roughly 320Vdc at 16 A? Suggestions? I am really confused here.

Bottom line is that I am so confused with this whole organization of how many panels should be in SERIES and how many SETS should be in PARALLEL before going to the inverter as one string or two strings and still use my 14-15 panels effeciently and have the system work. I think I have over read and looked at too many webpages to the point that its starting to get blurry. I have no idea of what other information to give so please let me know what I need to tell you or if you have a suggestion on connecting this setup and thank you.


Inverter INPUT (DC)
Max. DC power: 8300W
Max. DC voltage: 600V
Start voltage: 360V
DC rated voltage: 345V @208Vac and 379V @240Vac
PV voltage range: 300-600V @208Vac 345-600V @240Vac
MPP voltage range: 300-480V @208Vac 345-480V @240Vac
Max. input current: 30A
Max. input current per string: 30A
Number of MPP trackers/ strings per MPP tracker: 1/2

Inverter OUTPUT (AC)
Nominal AC output power: 8000W
Max. output current: 44A @208Vac 38A @220Vac
AC Nominal voltage/range: 208V / 183-229V, 240V /211-264v
AC grid frequency; range: 60Hz / 59.3-60.5 Hz
Power factor: 1
THDI: <3%
Consumption: operating (standby) / night: <5W / <0.5W

SOLAR PANELS (Multicrystalline cells)
Nominal Power (Pnom): 340 W
Power Tolerance: +5/−0%
Efficiency: 16.5%
Rated Voltage (Vmpp): 42.5 V
Rated Current (Impp): 8.00 A
Open-Circuit Voltage (Voc): 51.3 V
Short-Circuit Current (Isc): 8.52 A
Power Temp. Coef.: -174.4mV/° C
Voltage Temp. Coef.: 3.6mA/° C
Current Temp. Coef.: 1000V UL & 1000V IEC
Maximum System Voltage Maximum Series Fuse: 15A
 

Attachments

  • ds-sunpower-p17-340-commercial-solar-panels.pdf
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  • Renesola 8000 Grid Tie.pdf
    538.1 KB · Views: 3
  • Webster Solar.jpg
    Webster Solar.jpg
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  • Numbered Panel Layout.png
    Numbered Panel Layout.png
    209.6 KB · Views: 5

FilterGuy

What, me worry?
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Messages
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Location
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Hello! First timer here. I am trying to install my panels this April and I am ordering the PV cable and MC 4 connectors and I wanted to hear if I have my math correct or wrong please.

I am using a Rensola RePlus 8000TL Plus Grid Tie String Inverter and SunPower Performance Series SPR-P17-340-COM panels. I can use either 14 or 15 panels at this portion of the first install and I plan to add the others to bring the power as close to 8000w as possible so I have the choice of using 3 sets of 5 panels in series or 2 sets of 7 panels in series joined as the 1st string to the Inverter (as I believe it can also allow me to add a 2nd final string later (additional 3Kw of panels)).

I will attach the .pdf's of the panels and inverter but to be fast I did my best to enter the direct information for the specific models of each item. I hope this helps.

Quick history. I have my install permit from the county, the 8000w Inverter, 10Kw worth of 340w panels, mounting hardware, Combiner box, emergency disconnects and MC4 M&F connectors. I am getting ready to order Marine Grade 10AWG sheathed two wire (red and black), 2 to 1 and 3 to 1 MC4 combiners connectors, MC4 wrenches. I am debating on a thru the roof to the attic roof openning versus a long run of direct exposed cable (in proper outdoor gray plastic EMT pipe). I am open to any suggestions. I will try to post a rough drawing of my roof layout and limited panel placement and hopefully number the panels to make anyone's explanation easier to do. Thank you for reading this far as I am a long winded type of guy who tries to provide the most information the first time around.

My concerns are:
1.
I am trying to order some 2:1 and 3:1 MC4 pair branch connectors before the delivery starts getting worse with this Covid 19 (I want to get the order right the first time). I will be running 10AWG Marine Grade wire for all connections.

2. I don't understand the start voltage for the inverter. Does this mean that the rated start voltage of 360Vdc needs my string from 14-15 42.5Vdc panels to add up to at least the 360Vdc or more but not more than 600Vdc? I was thinking of doing three sets of five panels which has a combined series and final parallel voltage total of 212.5Vdc at 24 A. So I guess this does not work?

3. Should I focus on getting a series of panels to add up to at least 360Vdc and is the higher voltage the better up to 600Vdc? This may limit what I can put to the inverter as I was told that two strings going into the inverter should match voltage as close as possible between the two strings. Is this correct too?

4. With 15 panels, I can combine 8 for a series Vdc total of 340Vdc and combine the other 7 for a series Vdc total of 297.5 (all under optimum conditions of course) but this sounds like the inverter will not even start with this Vdc power incoming plus the two sets of incoming would be what ... roughly 320Vdc at 16 A? Suggestions? I am really confused here.

Bottom line is that I am so confused with this whole organization of how many panels should be in SERIES and how many SETS should be in PARALLEL before going to the inverter as one string or two strings and still use my 14-15 panels effeciently and have the system work. I think I have over read and looked at too many webpages to the point that its starting to get blurry. I have no idea of what other information to give so please let me know what I need to tell you or if you have a suggestion on connecting this setup and thank you.


Inverter INPUT (DC)
Max. DC power: 8300W
Max. DC voltage: 600V
Start voltage: 360V
DC rated voltage: 345V @208Vac and 379V @240Vac
PV voltage range: 300-600V @208Vac 345-600V @240Vac
MPP voltage range: 300-480V @208Vac 345-480V @240Vac
Max. input current: 30A
Max. input current per string: 30A
Number of MPP trackers/ strings per MPP tracker: 1/2

Inverter OUTPUT (AC)
Nominal AC output power: 8000W
Max. output current: 44A @208Vac 38A @220Vac
AC Nominal voltage/range: 208V / 183-229V, 240V /211-264v
AC grid frequency; range: 60Hz / 59.3-60.5 Hz
Power factor: 1
THDI: <3%
Consumption: operating (standby) / night: <5W / <0.5W

SOLAR PANELS (Multicrystalline cells)
Nominal Power (Pnom): 340 W
Power Tolerance: +5/−0%
Efficiency: 16.5%
Rated Voltage (Vmpp): 42.5 V
Rated Current (Impp): 8.00 A
Open-Circuit Voltage (Voc): 51.3 V
Short-Circuit Current (Isc): 8.52 A
Power Temp. Coef.: -174.4mV/° C
Voltage Temp. Coef.: 3.6mA/° C
Current Temp. Coef.: 1000V UL & 1000V IEC
Maximum System Voltage Maximum Series Fuse: 15A
OK.... A lot to cover, but I will concentrate on your string voltage.

1) Yes, the inverter will not start till it sees an input voltage of 360Volts. So your voltage needs to be higher than 360V, but your max voltage needs to be less than 600V.

2) When calculating your max string voltage you have to take into account the rise in voltage for low temperatures. There is a nice calculator for max voltage that @upnorthandpersonal posted here: https://diysolarforum.com/threads/maximum-open-circuit-voltage-calculator.4500/

The temp coefficient of 174.4mv/C is lower than most so it may not make a large difference, but you should check.

3) When laying out your panels, keep shading in mind. If part of the array will be shaded part of the day, try to keep the shaded part on one string, that way only one of the strings will be impacted.


Question: Are you at the coast? Why are you going with Marine grade cable? It is good stuff, but very expensive. Also, if the cable is going to be exposed to the sunlight, make sure it is UV resistant or you *will* have problems down the road.
 

KenInNC

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Joined
Mar 26, 2020
Messages
14
OK.... A lot to cover, but I will concentrate on your string voltage.

1) Yes, the inverter will not start till it sees an input voltage of 360Volts. So your voltage needs to be higher than 360V, but your max voltage needs to be less than 600V.

2) When calculating your max string voltage you have to take into account the rise in voltage for low temperatures. There is a nice calculator for max voltage that @upnorthandpersonal posted here: https://diysolarforum.com/threads/maximum-open-circuit-voltage-calculator.4500/

The temp coefficient of 174.4mv/C is lower than most so it may not make a large difference, but you should check.

3) When laying out your panels, keep shading in mind. If part of the array will be shaded part of the day, try to keep the shaded part on one string, that way only one of the strings will be impacted.


Question: Are you at the coast? Why are you going with Marine grade cable? It is good stuff, but very expensive. Also, if the cable is going to be exposed to the sunlight, make sure it is UV resistant or you *will* have problems down the road.
Thank you for your information. I will check any links soon.

1. So I guess 14 panels in series would give me 595Vdc and as much as I would like to add the last panel to it that would top the 600Vdc rating. Is this correct? Or is it rare to have panels actually producing what they say during peak noon summer sun?

2. I will check the chart. Is the number I have a good coefficient or bad? These are special panels.

3. I do leave near the ocean. Marine grade seems to be able to carry a little more amp rating for the AWG when compared to other qualities of wires. I prefer pure copper and tinned is fine. Price is not too bad at around 100’ for $80-100. I also have an old boat I will restore so good wire will not go to waste.
As far as the shading there is non on this section of the roof unless I use the 15th panel which would only see possible shade caused in late evening by the intersecting roof line. Sadly the future second string area of panels will be on the East facing roof so it will not get the full days sun. Should this cause an issue that would hurt the 14 panels facing South as the first string as it will get less light and will more than likely be less voltage due to less panels installed? I don’t want to take away from the system.

My panels are of a odd design. They were designed for commercial power and the cells are in rows so that if a shadow falls on one row portion it should not affect the other rows above or below it if I heard this correctly.

thank you again! Ken
 

FilterGuy

What, me worry?
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Messages
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Thank you for your information. I will check any links soon.
1. So I guess 14 panels in series would give me 595Vdc and as much as I would like to add the last panel to it that would top the 600Vdc rating. Is this correct? Or is it rare to have panels actually producing what they say during peak noon summer sun?
Whups..... lets back up a bit. You seem to be using Vmp to calculate the string voltage. You need to be using Voc. The peak voltage will occur when the batteries are full and the Charge controller shuts off the current. This appears to the solar panel as an open circuit. You are very likely to see the full open circuit voltage even though there is zero power being produced. The most you can do in series is 11 panels and even that might be too many if you get any cold weather.

I thought you were planning on doing two parallel sets of 7 panels in series. This would give you total Voc of 359 volts, well under the 600V limit. This would give you a total Imp of 16 Amps. (More on this later)

2. I will check the chart. Is the number I have a good coefficient or bad? These are special panels.
The voltage temp coefficient of -174.4mv/C is lower than most... and that is a good thing.
The solar panels are tested and specified at 25C. As it gets hotter, the performance goes down. As it gets colder the performance goes up. Most panels I have seen typically have a voltage coefficient in the 200-300mv/C. Since the coefficient for your panels is lower it means the voltage change (both ways) will be less. The link I pointed to has a calculator that allows you to input the specs of your panels and your lowest expected temp and it will give you the calculated Voc at the temp specified.
Note: If you are using 2 parallel sets of 7 series panels you are probably are still well under 600 volts even in the coldest conditions.... but use the calculator just to be sure.

3. I do leave near the ocean. Marine grade seems to be able to carry a little more amp rating for the AWG when compared to other qualities of wires. I prefer pure copper and tinned is fine. Price is not too bad at around 100’ for $80-100. I also have an old boat I will restore so good wire will not go to waste.

OK..... just make sure it can handle the UV if it is going to be exposed to the sun. Also, since you are dealing with very high voltage, makes sure it can handle the voltage.

As far as the shading there is non on this section of the roof unless I use the 15th panel which would only see possible shade caused in late evening by the intersecting roof line. Sadly the future second string area of panels will be on the East facing roof so it will not get the full days sun. Should this cause an issue that would hurt the 14 panels facing South as the first string as it will get less light and will more than likely be less voltage due to less panels installed? I don’t want to take away from the system.

I don't know the details about your inverter, but the spec sheet says 'Single MPPT controller'. The spec for the total input current is 30A and that is the same as the 30A spec for each input. That tells me that the two inputs are wired in parallel to each other. So.... assuming your east array is also two sets of 7 in parallel, you will effectively have 4 sets of 7. Purely by the panel specs, you would be driving 32 amps (over spec of the inverter) for a total of 9520W (Way over spec of the inverter.)

However, since the two sets of arrays are not aligned, they won't peak at the same time.... In addition, a lot of people never get max power out of their panels due to longitude and panel alignment. You might still be ok. Unfortunately, we are getting out of my area of expertise (my experience is with smaller arrays with dedicated Solar Charge controllers). Hopefully someone else on the forum might be able help.

My panels are of a odd design. They were designed for commercial power and the cells are in rows so that if a shadow falls on one row portion it should not affect the other rows above or below it if I heard this correctly.

Interesting, it sounds like the cells are arranged in series columns and parallel rows. As the sun hits the top rows in the morning the panel will start producing even if the bottom rows are still shaded.
 

KenInNC

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Joined
Mar 26, 2020
Messages
14
Whups..... lets back up a bit. You seem to be using Vmp to calculate the string voltage. You need to be using Voc. The peak voltage will occur when the batteries are full and the Charge controller shuts off the current. This appears to the solar panel as an open circuit. You are very likely to see the full open circuit voltage even though there is zero power being produced. The most you can do in series is 11 panels and even that might be too many if you get any cold weather.

I thought you were planning on doing two parallel sets of 7 panels in series. This would give you total Voc of 359 volts, well under the 600V limit. This would give you a total Imp of 16 Amps. (More on this later)


The voltage temp coefficient of -174.4mv/C is lower than most... and that is a good thing.
The solar panels are tested and specified at 25C. As it gets hotter, the performance goes down. As it gets colder the performance goes up. Most panels I have seen typically have a voltage coefficient in the 200-300mv/C. Since the coefficient for your panels is lower it means the voltage change (both ways) will be less. The link I pointed to has a calculator that allows you to input the specs of your panels and your lowest expected temp and it will give you the calculated Voc at the temp specified.
Note: If you are using 2 parallel sets of 7 series panels you are probably are still well under 600 volts even in the coldest conditions.... but use the calculator just to be sure.



OK..... just make sure it can handle the UV if it is going to be exposed to the sun. Also, since you are dealing with very high voltage, makes sure it can handle the voltage.



I don't know the details about your inverter, but the spec sheet says 'Single MPPT controller'. The spec for the total input current is 30A and that is the same as the 30A spec for each input. That tells me that the two inputs are wired in parallel to each other. So.... assuming your east array is also two sets of 7 in parallel, you will effectively have 4 sets of 7. Purely by the panel specs, you would be driving 32 amps (over spec of the inverter) for a total of 9520W (Way over spec of the inverter.)

However, since the two sets of arrays are not aligned, they won't peak at the same time.... In addition, a lot of people never get max power out of their panels due to longitude and panel alignment. You might still be ok. Unfortunately, we are getting out of my area of expertise (my experience is with smaller arrays with dedicated Solar Charge controllers). Hopefully someone else on the forum might be able help.



Interesting, it sounds like the cells are arranged in series columns and parallel rows. As the sun hits the top rows in the morning the panel will start producing even if the bottom rows are still shaded.
Hang in there... I am doing a Grid Tie not a battery charging setup. I will look at the info you mentioned with fresh eyes in the morning. Thank you again! This is great help. I did just order the 10AWG Marine UV grade but I plan to protect it properly anyways.
 

svetz

Works in theory! Practice? That's something else
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I'm sure you can make it work; but if in doubt ask because as you've discovered there are a lot of folks happy to answer questions and rooting for you!
Be sure to post pictures and blog your experiences in the show and tell section as you go. It'll not only help others but folks might see something you missed!
 

KenInNC

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Odd question as I am getting confused about combining series to parallel totals for one string. I am trying to make sure that I have the necessary amount of panels to ensure the Inverter's Start Voltage of 360V is met and not input the maximum DC voltage of 600V in the winter by using the temperature coefficient calculator mentioned here to keep under the max DC input.

So my 8000W Inverter needs 360V to start with a maximum of 600V. My panels are 340W with Vmpp of 42.5V (use this times to get the minimum necessary panels in series just to start the inverter and probably needs some more for those lower light days?) and Voc of 51.3V (to use in the temp coefficient calculator).

So if I am limited to 14 -15 panels facing direct South at this time and any additional panels will face East and not South like the first 14-15 panels ...

One of my other concerns is how can I use the 14-15 panels in a way that will not hurt the inverter? I don't understand that if a Series of 10 has 42.5Vmpp which would be 425.0Vdc and the other lets say 5 panels would have 212.5Vdc which when the two strings are put in Parallel the combined voltage would be what? I don't know how you add two different Vdc in Parallel and maybe it is too strong at this point due to the 600Vdc Max input and the Temp co-efficiency math?

Here is the calculator info and I am not sure what numbers I need to use when adding Voltage for the Series for Starting Inverter and Over Powering Inverter:

You entered:
  • Total string OC voltage: 513 (10 panels in Series at Voc 51.3V each which totals 513V)
  • Nominal maximum power: : 340 (1 panel at 340W or is this a 10x 340W?)
  • Worst case temperature: -5. (Wilmington, NC average lowest (20F) in Celsius)
  • Temperature coefficient (Voc): -174.4 (Is this the Voltage Temp. Coef. of -174.4mV/C the brochure states?)
  • Temperature coefficient (Pmax): 3.6. (Is this the "Current Temp. Coef" the brochure states?
For these, your:
  • Total worst case differential is: 30
  • Your voltage increase (%) is: 5232.
  • Your voltage increase (V) is: 26840.16
  • Your maximum voltage (V) is: 27353.16

  • Your Pmax increase is (%): 108 This makes it sound like the 340W panel will produce 340W x 1.08%
  • Your Pmax increase is (W): 367.2
  • Your total Pmax at this temperature is (W): 707.2
If I entered in Sting OC Voltage as one panel only of 51.3V and one panel of 340W it gives this result which doesnt make sense as to why only two numbers changed and by the way ... what is correct? The OC Voltage of 513V or 51.3V for a series of 10 panels?:
  • Total worst case differential is: 30
  • Your voltage increase (%) is: 5232
  • Your voltage increase (V) is: 2684.016
  • Your maximum voltage (V) is: 2735.316

  • Your Pmax increase is (%): 108. (So 108% of the original Voc of 51.3V give me 59.4V. So 10 panels in series Voc would be 594V?)
  • Your Pmax increase is (W): 367.2 (Wattage actually increases above the stated Pnom of 340W to 367.2W?) (Shorter daylingt and colder days but more Watts?)
  • Your total Pmax at this temperature is (W): 707.2 (Do I need to use this number?


I feel that my ignorance in looking for a deal in the December of 2018 (to get the tax credits) has put me in a difficult matching place for the panels and inverter based on my roof direction and limited mounting space. I ended up buying 30- 340W panels for about $3,400 shipped and another $400 for the 8000KW Grid Tie Inverter. Add the other items like mounting hardware and wire and my system is around $5,300 invested but of course I can't use all of the panels at this time. When I hear of the people who spent $20k plus it seemed like over the long run the system would finally pay off when the system was in its 20th year or so.
 

svetz

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... I am getting confused about combining series to parallel totals for one string...Here is the calculator info and I am not sure what numbers I need...
Read through these two posts and work though the examples:
Calculators are great, but understanding how things work is better as you'll be able to calculate the answer on your own and not have doubts about what the inputs should be. It'll also help you diagnose the eventual future problems. The first post will tell you how voltage and amperage combine with panels in series and parallel. The second will tell you about how they work with an MPPT and your weather.

A "string" is a series of panels. Multiple strings are wired together in parallel. When you see 4s2p, it means two parallel strings of 4 panels in series.

Your MPPT's starting voltage is 360V and max is 600V. It sounds like the inverter has two PV inputs, rated for up to 30 Amps each.
Your panels are 51.3 V & 8.52 A and Voltage Temp. Coef.: −173.7 mV / °C and Current −3.6mA / ° C (You had these reversed)
So, 360V / 51.3V rounded up is 8. That's the minimum number of panels per string.
For the maximum you need to consider the temperature correction factor.
The coldest temperature on record for for Wilmington, NC is -18 Celsius ... not -5. So...

Voc @ -18C = 51.3 (Voc @ STC) + (−173.7mV / (1000 mv/V) x (– 18°C - 25°C)) = 58.739V

So, 600 / 58.739 rounded down = 10, that's the maximum safe number of panels per string.

How many strings can go onto a single MPPT input? 30 Amps.

Each string is 8.52 Amps. At -18°C that's corrected to 8.6748 A. SO, 30A / 8.52A rounded down is a max of 3 strings in parallel.

Definitely DO NOT trust my math... while I'm handy with the theory using a calculator properly is somewhat beyond me.

...if I am limited to 14 -15 panels facing direct South

15 panels in series would have a maximum potential voltage of 15x58.739(Voc@-18°C) = 881V, way to high. ❌
27 Panels configured as 9s3p would give 528V and 26 amps. ✅

What would 30 Panels configured as 10s3p would give you?
 

KenInNC

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Thank you so much. I have a lot more studying to do. I believe I had read one of the listed recommendations but I will check again. Again, thank you. Sometimes I think the nomenclatures used here and over in Asia might seem differently worded. I realize that the terms like Voc shouldn’t make a difference but some seem difficult to get currently for myself.
So the 9s3p sounds like a plan but if I can only do a single 10s combined with a 5s because I am not ready for the other 12 panels, then this is where my issue with combining two different voltage outputs in parallel comes in. I will read the articles in hopes that it will cover this. I do hope they cover the goods and or bads of mixed voltage combinations. I will try to use that calculator again. Thank you for teaching and advising me and I understand the not responsible clause ... 😁
 

KenInNC

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Some guy named Svetz 😁 is the reason why I joined this forum. I was searching circuit info which led to your well illustrated article “Solar panel strings: Parallel & Series explained”.

I still need to understand about combining different DC voltages for the different voltages of each set of not the same quantity series panels to a string. I am working on the research for this. Thank you
 

KenInNC

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Ok I had read both of the articles before I started the original post. It helped a lot but it doesn’t always sink in.
I guess the best thing I can do is just do a 9s1p South facing for now and connect the other 5s (South facing) with the 4s (East facing) which only get full power for 1/2 the day and finally when that second set is connected I would add the final third set of 9s1p (East facing) and that’s as good as I can get it. I wish I had better roof options but it is what it is. I don’t want to do a ground rack as it would eat up too much land.
 

KenInNC

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In the final build it should be like this:

27 Panels configured as 9s3p for 528V and 26 amps. And I think this post is done unless I am missing something
 

KenInNC

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Wow I am an idiot... if my inverter is 8000w then how can it handle the Pnom of 27x340w= 9,180w or just because the 9s3p voltage is 528v at 26a everything is ok? A watt calculator for DC said that the 528v and 26a would equal 13728 watts???

17a @480v (8s2p) = 8000w yet based on the panel Pnom 340w x 16 panels = 5440W ??? I am so lost
 

svetz

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One other thing to watch for, it looks like you have two MPPTs inputs; a lot of of those all-in-ones want the power balanced between them (e.g., same number of panels on each).

...Some guy named Svetz 😁 is the reason why I joined this forum...
He's okay on theory, but practice? Pffft! ;) Happy to be of assistance and hope this project works out well for you. Do be extremely careful with that high voltage, dangerous stuff! Remember that you can DIY bolt everything down, and hire the electrician to do the"wiring" at their flat hourly rate.

Wow I am an idiot... if my inverter is 8000w then how can it handle the Pnom of 27x340w= 9,180w [of panels]
It's actually pretty common to have 10-15% more solar than inverter because the panels only put out max watts around solar noon. Sure, it clips power at solar noon for a few days out of the year when the panels are perfectly aligned, but you have more power overall when the panels aren't perfectly aligned. Just don't exceed the max inputs, otherwise you can hurt the MPPT/system. For example, the Sol-ark 8k inverts 8kW, but can handle up to 11kW panels.

You can get clipping data by putting the data into SAM. SAM will also let you know if you have design problems. A steep learning curve for it, so I suggest watching the tutorial videos on it.

...17a @480v (8s2p) = 8000w yet based on the panel Pnom 340w x 16 panels = 5440W ??? I am so lost...

You'll find it really helps to keep labels/dimensions on your calculations. Let's add them to your math and see if it helps....

Voc @ -18° C = 58.739V
Imp @ -18°C = 8.6748 A

So 8s2p @ -18°C:
  • V-18°C = (58.739V * 8) = 470 V
  • I-18°C = 8.6748 A x 2 = 17.4 A ;
  • W-18°C =470V x 17.4A = 8154 W ✅
16 Panels @ 340WSTC = 5440 W ✅
 
Last edited:

Hedges

I See Electromagnetic Fields!
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Mar 28, 2020
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One other thing to watch for, it looks like you have two MPPTs inputs;

I read the inverter spec summary as saying one single MPPT input, and you could connect two strings. Of course you can connect more so long as you don't exceed current rating (30A) and you fuse them. The panels are 8.5A short circuit current so 3 would be OK. All strings ought to be the same number of panels. Each string can be oriented differently (in which case you might get away with 4 strings and not exceed 30A. Some inverters have separate "short circuit" and "operating" max currents.)

Checking open circuit voltage and temperature coefficient I thought 10 panels in series would be good, down to -15C without exceeding 600 Voc.
7 panels would not be enough, only about 300V mpp in full sun. Any less light and the MPPT couldn't go lower to track it.

Those SunPower P17 Performance series are "different". I considered them but went with the E series instead. Two reasons: One, new assembly method, conductive adhesive joining cells that are shingled over each other. I wanted to stick with something proven over decades. Two, those stripes meant to tolerate shading, assuming all panels in a string are shaded identically (commercial ground array, each row shading the next when sun is low in the sky.) It isn't clear they even have any bypass diodes at all. Not mentioned in the data sheet. Make sure they don't get any shading at all. Read documentation or take measurements to check for diodes, and consider adding a bypass diode per panel if not built in.
 
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