EG4 18kPV - String Calculations

[WattAboutThat]

EG4 - 18kPV - String Calculations



Using Canadian Solar 385W Panels



This is my first time attempting to calculate

PV String series to determine maximum panels

i can safely put in a string and not exceed max voltage of the MPPT input.



Many of my assumptions may be incorrect,

and I am seeking feedback, to determine the safe quantity of panels in series per mppt.



——

Knowns:



——

18kPV Inverter Specs



per mppt

max volts 600V

max current 25A/15A/15A

max input 18kW



——

Canadian Solar Panel Specs



Wattage: 385W

VOC: 43.9V

ISC: 11.32A

Low-temperature coefficient (Pmax):

-0.34 % / °C



——

Assumptions:



I will design to use each mppt string

so all mppt strings are the same,

so designing to only use up to 15A on mppt1

so the i have the same size strings on each mppt.



Will assume max below zero temperature

for calculations to be -30F

Never has been that cold in Indiana to my knowledge, so just a safety margin.



——

Voltage Calcs using Temp Coeficient



Using Assumed -30F below Zero



(-30°F − 32) × 5/9 = -34.44°C



25C - (-34.44C) = 59.44C difference (delta)



43.9V x .34% = .14926V per degree C



.14826V x 59.44 = 8.8125744V per module



43.9V + 8.7125744V = 52.7125744V



in string of 10series



527.125744V per string



which is less than 600V



——

Current (Amps) Calcs



1 String (no paralleling) = 11.32A

which is less than 15A max



——

Watts per String

527.125744V x 11.32A = 5,967.06342W



x3 strings = 17,901.1903W

which is below 18kW of total input



——

Conclusions:

So assuming all of my calculations and assumptions are reasonably correct,

I believe it would be safe to install

3 Strings of 10 Canadian Solar Panels

one string per each mppt,

and remain safely below max V, max W, and not clip max A.



——

Please advise of any corrections
I should consider.

Thanks

 

[Plum Crazy Rob]

Your basic calculations sound ok?? Hopefully an expert will offer additional considerations.

 

[FilterGuy]

-0.34 % / °C

Low-temperature coefficient (Pmax):

-0.34 % / °C

OOPS! Wrong coefficient. You need the Voc temp coefficient. Sometimes it is called the beta. My guess is it is a smaller value so you will probably find 10 in series is still ok.

 

[FilterGuy]

BTW: I looked up a few 385W Canadian Solar panels. I could not find one with a Power coefficient of -.34, but they all had a voc coefficient of -.29

 

[Plum Crazy Rob]

-.26
GOOD call.

I think these:

 

[FilterGuy]

with a beta of -.26, I calculate a string voltage of 506.84 for 10 panels.

BTW: The following resource has a table in the back that simplifies the math.

Adjusting Solar Panel Voc for temperature

When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...

diysolarforum.com

 

[SignatureSolarJames]

EG4 - 18kPV - String Calculations



Using Canadian Solar 385W Panels



This is my first time attempting to calculate

PV String series to determine maximum panels

i can safely put in a string and not exceed max voltage of the MPPT input.



Many of my assumptions may be incorrect,

and I am seeking feedback, to determine the safe quantity of panels in series per mppt.



——

Knowns:



——

18kPV Inverter Specs



per mppt

max volts 600V

max current 25A/15A/15A

max input 18kW



——

Canadian Solar Panel Specs



Wattage: 385W

VOC: 43.9V

ISC: 11.32A

Low-temperature coefficient (Pmax):

-0.34 % / °C



——

Assumptions:



I will design to use each mppt string

so all mppt strings are the same,

so designing to only use up to 15A on mppt1

so the i have the same size strings on each mppt.



Will assume max below zero temperature

for calculations to be -30F

Never has been that cold in Indiana to my knowledge, so just a safety margin.



——

Voltage Calcs using Temp Coeficient



Using Assumed -30F below Zero



(-30°F − 32) × 5/9 = -34.44°C



25C - (-34.44C) = 59.44C difference (delta)



43.9V x .34% = .14926V per degree C



.14826V x 59.44 = 8.8125744V per module



43.9V + 8.7125744V = 52.7125744V



in string of 10series



527.125744V per string



which is less than 600V



——

Current (Amps) Calcs



1 String (no paralleling) = 11.32A

which is less than 15A max



——

Watts per String

527.125744V x 11.32A = 5,967.06342W



x3 strings = 17,901.1903W

which is below 18kW of total input



——

Conclusions:

So assuming all of my calculations and assumptions are reasonably correct,

I believe it would be safe to install

3 Strings of 10 Canadian Solar Panels

one string per each mppt,

and remain safely below max V, max W, and not clip max A.



——

Please advise of any corrections
I should consider.

Thanks

Pretty good math, the thing is that you can go up to 21kw so you are more comfortably under the limit than you may think

The 600v max really helps in cold climates

 

[FilterGuy]

You could do 2 strings of 11 on MPPT 1, 1 string of 11 on MPPT 2, and 1 string of 11 on MPPT 3. 44 panels total.

44 385W panels would be 16.94KW.... Still plenty of headroom! Even if you only had an insolation number of 3 you would get 50KWh/day!

 

[WattAboutThat]

I want to thank each of you above for your very valuable info. All of which I would have missed or overlooked. I truly appreciate your mentoring me. And hopefully in the future when I am more knowledgable, I will be able to repay your kindness.

 

[Plum Crazy Rob]

Also note: full power MPPT range of 230-500v so keep nominal/typical design operating conditions within that right??

 

[FilterGuy]

Also note: full power MPPT range of 230-500v so keep nominal/typical design operating conditions within that right??

When it goes above 500 (but still under 600), it doesn't track any longer, but it is still producing lots of power. As far as I know, there is no damage between 500 and 600.

Think of it this way: You can panel to 500Voc STC and use the extra 100V to deal with cold temp voltage rise. That allows you to max out the MPPTs.
(I hate it when I have to leave a lot of margin on the MPPT because of the cold temp voltage rise).

 

[FilterGuy]

Think of it this way: You can panel to 500Voc STC and use the extra 100V to deal with cold temp voltage rise. That allows you to max out the MPPTs.
(I hate it when I have to leave a lot of margin on the MPPT because of the cold temp voltage rise).

My statement should have said

"You can panel to 500V Vmpp and use the extra 100V to deal with cold temp Voc."

With the 385W panels that @Plum Crazy Rob shared, there is an extra 7.3V per panel between Voc and Vmpp. At -30F, you can have up to 11 of the panels for a temperature-adjusted Voc of 557.3V but the temperature-adjusted Vmpp is only 464.6V.... well within the MPPT range. I have not run the numbers, but you could probably use 11 of the higher wattage panels that are shown in the spreadsheet and still be inside both the Voc and Vmpp limits.

At -10F, you can do 12 of the 385W panels in series and the numbers work out to be temp-adjusted Voc of 593V and temp-adjusted Vmpp of 494V. That is a near-perfect match to the controller!

 

[Plum Crazy Rob]

My statement should have said

"You can panel to 500V Vmpp and use the extra 100V to deal with cold temp Voc."

Got it. Thanks for clarifying, now I understand the relationship ?

 

[rakitejada]

Thanks for all your help. I love the forum.

This is also my first time calculating each string for an mppt and i need your help.

I am a little confused regarding Mppt 1 and its two inputs and utilizing the best i can the power i can get.

I have three panel options:

A) JINKO 575w. 50.88VOC. 14.39ISC

B) JASOLAR 555w. 50.02VOC. 14.07ISC

C) CANADIAN SOLAR 550w. 49.8VOC. 14.05ISC

So MPPT 2 and 3 are pretty easy since they are 15AMPS MAX and 1 input:

11 JINKO 575w in series would be safe on VOC (559.68VOC) and with 14.39ISC I think I am safe. That would bring in: 575w x 11 = 6,325w on each MPPT x 2 = 12,650w

Now, going back to my initial confusion: MPPT 1: I havent found data on how those two inputs add up: should i assume they are in parallel or in series? I know they must be a simple question for many of you but really don’t know how to calculate.

Im thinking i can afford around 15,000w of panels right now and if i need the extra 6,000w i can do in the future, but have everything set up and wired so in the future its just an easy add on.

I live in the carribbean and i dont need to take in consideration winter or cold because its summer all year long.

Lastly, the reason i would like the extra wattage is because my wife and I own a Tesla and a Chevy Bolt, since i work from home it would be great to charge during sunlight hours and i also bought 6 EG4 Lifepower Batteries x 5.12kWh = 30.72kWh that will also be charging during sunlight.

I wont be able to inject the grid until I get all the permits and that usually takes 6 months. So i’ll be making my best effort to work around charging the cars in the morning along with Lifepower batteries and saving energy in the night to see if i can make it through the night with the 30.72kWh from the batteries (3 Inverter A/C’s 2*12,000btu and 1*18,000btu) but most likely I know I’m going to end up using some power from the grid.

What are you recommendations?

Thank you so much for your time!

Raki

 

[rakitejada]

Thanks for all your help. I love the forum.

This is also my first time calculating each string for an mppt and i need your help.

I am a little confused regarding Mppt 1 and its two inputs and utilizing the best i can the power i can get.

I have three panel options:

A) JINKO 575w. 50.88VOC. 14.39ISC

B) JASOLAR 555w. 50.02VOC. 14.07ISC

C) CANADIAN SOLAR 550w. 49.8VOC. 14.05ISC

So MPPT 2 and 3 are pretty easy since they are 15AMPS MAX and 1 input:

11 JINKO 575w in series would be safe on VOC (559.68VOC) and with 14.39ISC I think I am safe. That would bring in: 575w x 11 = 6,325w on each MPPT x 2 = 12,650w

Now, going back to my initial confusion: MPPT 1: I havent found data on how those two inputs add up: should i assume they are in parallel or in series? I know they must be a simple question for many of you but really don’t know how to calculate.

Im thinking i can afford around 15,000w of panels right now and if i need the extra 6,000w i can do in the future, but have everything set up and wired so in the future its just an easy add on.

I live in the carribbean and i dont need to take in consideration winter or cold because its summer all year long.

Lastly, the reason i would like the extra wattage is because my wife and I own a Tesla and a Chevy Bolt, since i work from home it would be great to charge during sunlight hours and i also bought 6 EG4 Lifepower Batteries x 5.12kWh = 30.72kWh that will also be charging during sunlight.

I wont be able to inject the grid until I get all the permits and that usually takes 6 months. So i’ll be making my best effort to work around charging the cars in the morning along with Lifepower batteries and saving energy in the night to see if i can make it through the night with the 30.72kWh from the batteries (3 Inverter A/C’s 2*12,000btu and 1*18,000btu) but most likely I know I’m going to end up using some power from the grid.

What are you recommendations?

Thank you so much for your time!

Raki

Anyone?

 

[FilterGuy]

I havent found data on how those two inputs add up: should i assume they are in parallel or in series?

Parallel

I live in the carribbean and i dont need to take in consideration winter or cold because its summer all year long.

JINKO 575w

Are those the Tiger Neo N-type 72HL4-(V) 555-575 Watt panels? They have the lowest Voc temp coefficient I have ever seen.

With 11 in series, You could get down to about -3c before it becomes a problem.

For MPPT 1 with 2 strings of 11, the Isc would be 2 x 14.39 = 28.78A. This is well under the 31A Max Isc. The Imp would be 2 x 13.62A = 27.24A but the MPPT 1 Usable current is only 25A. This means that if your panels produce at the full STC rating there might be some clipping at mid-day. However, there is a reasonable expectation the panels will not actually produce at full STC and there will be no clipping.

The reasons they may not produce at full STC include imperfect angle to the sun, and loss in production due to heat, shading, and aging.

 

[ksmithaz1]

IMNSHO you really don't want to build anything up against the edge of a spec. Further I found the problem is how many panels I can cram in the space. To keep them even I ended up with 32 so that factors down to 4 strings of 8 panels. VOC = 50 (48.something) So figure 400V/string @ <11A (455W panel). 400V is well above the 120 minimum and well below the 600v max. Makes all the temperature calculations moot. Find a happy spot in the middle and use it. Stay away from the edges, don't overthink it.

 

[moogambo]

Hi everyone, I also have the 18kpv and thought I had my strings figured out and after more reading and talking to an installer I am even more confused. Mppt 1 is the one that throws me off. It's parallel. and 25 amps. below is my set up.
Ground Mount.

31 count Bluesun BSM 550M-72HBD Bifacial Panels
VOC 49.92
VMP 39.1
Coefficient VOC -0.26
For my area the installer said -23 c I found the coldest ever was -32c

It was also suggested that I use Tigo Optimizers. Either way I am trying to figure the best way to string these panels. I was told I could do 8 and 8 on MPPT 1
Mppt 2- 8
Mppt 3- 7

Mppt 2 and 3 would be in series, correct?

Mppt 1 is again where I am lost in parallel wouldn't I be way over the amps allowed.
By the way I also have 30.72Kwh of batteries.

Any help would be greatly appreciated.
Thank You

 

[pvgirl]

Hi everyone, I also have the 18kpv and thought I had my strings figured out and after more reading and talking to an installer I am even more confused. Mppt 1 is the one that throws me off. It's parallel. and 25 amps. below is my set up.
Ground Mount.

31 count Bluesun BSM 550M-72HBD Bifacial Panels
VOC 49.92
VMP 39.1
Coefficient VOC -0.26
For my area the installer said -23 c I found the coldest ever was -32c

It was also suggested that I use Tigo Optimizers. Either way I am trying to figure the best way to string these panels. I was told I could do 8 and 8 on MPPT 1
Mppt 2- 8
Mppt 3- 7

Mppt 2 and 3 would be in series, correct?

Mppt 1 is again where I am lost in parallel wouldn't I be way over the amps allowed.
By the way I also have 30.72Kwh of batteries.

Any help would be greatly appreciated.
Thank You

Simplest setup is 10 in series on each MPPT and save 1 panel as a spare. Running two strings in parallel on MPPT 1 pushes you close to the 31 amp short circuit current limit, which is not helped as these are bifacial panels the gain from this will push the current higher and very close to this limit. 10 in series is safe to -43c/-45f, and is the most you want to have in series at your temperatures.

The need for optimizers will depend on the potential for shading in the location of you ground mount array.

 

[FilterGuy]

Hi everyone, I also have the 18kpv and thought I had my strings figured out and after more reading and talking to an installer I am even more confused. Mppt 1 is the one that throws me off. It's parallel. and 25 amps. below is my set up.
Ground Mount.

31 count Bluesun BSM 550M-72HBD Bifacial Panels
VOC 49.92
VMP 39.1
Coefficient VOC -0.26
For my area the installer said -23 c I found the coldest ever was -32c

It was also suggested that I use Tigo Optimizers. Either way I am trying to figure the best way to string these panels. I was told I could do 8 and 8 on MPPT 1
Mppt 2- 8
Mppt 3- 7

Mppt 2 and 3 would be in series, correct?

Mppt 1 is again where I am lost in parallel wouldn't I be way over the amps allowed.
By the way I also have 30.72Kwh of batteries.

Any help would be greatly appreciated.
Thank You

This is the 18Kpv PV specs.



31 panels @ 450W is 13.95KW. That is below the max usable power... so that is fine.

For 8 panels (Single or double string) the VOC @ STD is 49.92 x 8 = 399V. This is well below the 600V max of the MPPTs. It is far enough below 600V that without even doing the calculation I can confidently say there is no cold temp issues. Furthermore, it will remain well within the MPPT voltage operating range.

Could you please provide Isc and Imp for the panels? We also need to know what the backside gain is. (My guess is it will increase the Isc and Imp proportional to the power gain... and they will show a gain of up to 25% )