Figuring out how many panels in series and parallel based on your MPPT

svetz

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Update: See Will's Video on the topic.

MPPTs and String inverters can be quite perplexing at first as you try to figure out what the panel configuration should be. If you're not sure what's meant by serial and parallel in regards to solar panels check this link out first.

Easiest of course is to use a program like SAM (a big learning curve, but let's you play with a lot of what ifs).

For this example let's use a DUAL 2X 80A 3-stage MPPT with a max PV input 145V

Because this MPPT has a dual input, we'll want at least two identical parallel strings (or multiples of 2) so the power is balanced on each input.
So, let's run through an example!

Let say you take a shine to Panasonic's 330W HIT panels. So you google to find the datasheet and look up a few things:

Voc = 71V​
Isc = 6.08​
Temperature Coefficient (Voc) = -0.17C​

You want 5 kw and the panels are 335W, so divide it out (5000/335) and that's 14.92 panels. They don't sell fractional panels, so you could round up to 15.
But alas! You want to have an equal number of panels on each string, so let's bump it up to 16!

So, 16 panels/2 strings = 8 panels in series per string. Each panel has 71V, so with 8 panels that's 8x72= 576V -- OOPS! 576V is way over the 145V maximum of the MPPT... That's not going to work!

The voltage on the Panasonic's is fairly high; what would happen if you used something with a lower voltage like the LG Neon2's? Here's that datasheet:

Voc = 41V​
Isc = 9.8​
Temperature Coefficient (Voc) = -0.27C​

So two strings of 8 panels would give you 328V and 9.8 amps each. Nope... Well, how about 4 parallel strings of 4 panels? Nope, each string is 164V. Still too high! Could you make it work with two strings of 8 panels in parallel? That would be 34V and 78.4 amps... that's just under the limit. It would work, but how about an even lower voltage?

How about those AltE 150W panels? To get 5000W we'd need 30 panels (4.5kW)! 30 panels is nearly twice 16, that's a lot more racking and wire so it's gonna cost more!

Voc = 22.32V​
Isc = 8.86​
Temperature Coefficient (Voc) = -0.32%/C​

With 30 panels we could do 6 parallel strings of 5 panels in series, 5x22.32 = 112V! Whew, made it! 112 < 142! Sweet, so you'd put 3 strings on the first MPPT input and 3 on the other.

But what about the amps? Each MPPT can take 80 amps, and string is 8.86 amps, so when 3 are combined it would be under 27 amps...no problem there!

Or is there?

Temperature Correction
What's the coldest on record it's been there? Your county will have a magic number for this and you'll have to meet it in order to get your system approved (There's a similar "hot" number for when you get to calculating the wiring).

You see, the colder it gets the more power the panel produces. If you're in Colorado it might be -40°C. So, that's a temperature delta from STC of 65°C (25°C - (-40°C)), Which we multiply by the Temperature Coefficient, so 65°C x .32=20.8%

(100% + 20.8%) x 22.32 Voc ~= 27 Voct, and with 5 panels in series 27 x 5 = 135v Whew! Made it! Less than 142V on the coldest possible day of the year!

There's also a temperature coefficient for current, but it works the same way.

Hopefully, this was both entertaining and informative for how you can figure out your panel configuration based on your MPPT settings, the panels you pick, and local conditions.
 
Last edited:

buzzlightyear

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Is it preferred/required to balance two strings on two different mppts on the same inverter? I was thinking of putting a string that faces east on one mppt to start early in the morning, and the main part of the array that faces north (yes I'm in the southern hemisphere) on another mppt.
 

svetz

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For 240V inverters with MMPTs, I suspect the reason they like you to balance is that the first one goes to L1 and the second to L2.
If it was dual input on the same MPPT and the manufacture specifies to balance it's probably some internally shared element.

Haven't done it, but for totally separate MPPTs I can't see why they would care how much of a load the other MPPT was carrying as long as the wires to the battery were relatively short and close to the same length. I think in fact that what you want to do is the best way to do it.
 

koppted

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At the risk of sounding stupid I got a question on your first example:

Say I have 14 of the the Panasonic's 330W HITs
Voc = 71V
Isc = 6.1 A?
DUAL 2X 80A 3-stage MPPT with a max PV input 145V


Can i connect 2 panels in series to make 7 pairs(counting as 1 set), and then connect the sets in parallel?:
So for example 2 Panels in series mean
Set 1 142V at 6.1A
Set 2 142V at 6.1A
Set 3 142V at 6.1A
Set 4 142V at 6.1A
Set 5 142V at 6.1A
Set 6 142V at 6.1A
Set 7 142V at 6.1A

Connect the sets to each other in parellel would mean 142V at 42A, is this doable with the 80A MPPT charge controller?
What would be the drawbacks of such a setup?
 
Last edited:

gnubie

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If your MPPT can handle at least 150v on the input that would work fine in most cases. If you live in an environment where it gets really cold you can find the voltage on your panels exceeds 71VoC so you'd have to get a higher spec'd controller if that is the case.

My own panels are arranged that way to keep the array VoC below 100v. The trade off is the current. You need heavier wire to safely carry the current, but also to avoid higher losses. 42amps may well exceed the rating of your MC4 connectors if the output of the combiner / input of the charge controller uses MC4 connectors.
 

koppted

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If your MPPT can handle at least 150v on the input that would work fine in most cases. If you live in an environment where it gets really cold you can find the voltage on your panels exceeds 71VoC so you'd have to get a higher spec'd controller if that is the case.

My own panels are arranged that way to keep the array VoC below 100v. The trade off is the current. You need heavier wire to safely carry the current, but also to avoid higher losses. 42amps may well exceed the rating of your MC4 connectors if the output of the combiner / input of the charge controller uses MC4 connectors.
Follow up, is that just a limitation of MC4 connectors in general or are these limitations manufacturer specific?

A Charge controller Like this one rated for 100Amps 60-155Vdc@48V : https://www.amazon.com/Aims-Power-SCC100AMPPT-Charge-Controller/dp/B078VL7HGX

Will they work for setup I list above, if I have connectors that don't have Amperage limitations you mentioned?
 

koppted

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I just read up on MC4 connectors apparently they are "UL rated at 20 A and 600 V" So i guess that answers one part of my question.
 

gnubie

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I was just typing something along those lines. There are heavier built MC4 connectors that will handle more current but you have to be careful what you buy. If it comes from the usual online sellers from China the ratings usually won't match the actual product.

The charge controller has nice big screw terminals so no problem there with current.
 

MrNatural22

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I just read up on MC4 connectors apparently they are "UL rated at 20 A and 600 V" So i guess that answers one part of my question.

XT90 connecters can handle 10g wire @ 90amps 200 v easier to connect-disconnect but, they are not weather proof like MC4
 

Dnnap

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Temperature Correction
What's the coldest on record it's been there? Your county will have a magic number for this and you'll have to meet it in order to get your system approved (There's a similar "hot" number for when you get to calculating the wiring).

You see, the colder it gets the more power the panel produces. If you in Colorado it might be -40C. So, that's a temperature delta from STC of 65 (25 - (-40)), Which we multiply by the Temperature Coefficient, so 65x.32=21V

21V + 116V = 138V Whew! Made it! Less than 142V on the coldest possible day of the year!
There's also a temperature coefficient for current, but it works the same way.

Hopefully this was both entertaining and informative for how you can figure out your panel configuration based on your MPPT settings, the panels you pick, and local conditions.

I have a question pertaining to the calculation of the temperature correction based on the given temperature coefficient for VOC referenced in the above quote.
In lieu of the values described in the above quote, shouldn’t the calculation be as follows?

Given:
- VOC = 116V
- Temperature coefficient = -0.32% per °C
- Temperature change = (25°C - (-40°C) = 65°C
- Total percent deviation = 65°C x 0.32 = 20.8%

Voltage change = 116v x 0.208 = 24.128V
VOC correction = 116V + 24.128V = 140.128V or 140v at -40°C.
 
Last edited:

Crisbarcco

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PLEASE HELP!!!
I live in east Hawaii county. My home was taken by lava on 2018. So, I finally I am done building a new cottage. Now it’s time for me to install the solar panels, batteries, and MPPSolar inverter.
So I saw these 340w new panels for $165 (good deal! since here everything is more expensive here) i was planning to buy 6 for a 2040w array.
Pmax 340w
Vmp 38.2v
Imp 8.90a
Voc 46.2v
Isc 9.50v
I am not sure if I can make these panels work with the new series of “all in one” MPPSolar 3000w 48v inverter. (Spect sheet picture below) (65 dregree it’s the coldest it ever get here)
Mppsolar 3000w 48v
Solar charger mppt
Max pv array power 4000w
Mppt range @operating volt 60~115v
Max pv array open circuit volt 145v
Max solar charge current. 80a

I thought that I can do 3 parallel strings of 2 panels in series. 2x46.2 =92.4v

But after reading this post above,I understand that it’s better for me to connect 2parallel of 3 panels in series 3x46.2 138.6v just under 145vdc???

Or these panels are not gonna work at all?


I gladly appreciate any advice since I don’t want pass this deal of inexpensive panels.
 

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Supervstech

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Is shading an issue? If not, 3s is better because the transmission wire needs to carry less amps, so it scan be smaller conductors.
 

Crisbarcco

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Shade isn’t an issue. 3 In series it is. Thank youfor the advice!

The part that is about difficult to understand is the open circuit limitation. I don’t think I can return these panels once I pay for them. So I wanted to be sure that I can use them. Thank you again spervstech!
 

Supervstech

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If the charge controller can handle 145V, and the max open circuit voltage is below that, then yes. You can.
 

gzoro

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Update: See Will's Video on the topic.

MPPTs and String inverters can be quite perplexing at first as you try to figure out what the panel configuration should be. If you're not sure what's meant by serial and parallel in regards to solar panels check this link out first.

Easiest of course is to use a program like SAM (a big learning curve, but let's you play with a lot of what ifs)/

For this example let's use a DUAL 2X 80A 3-stage MPPT with a max PV input 145V

Because this MPPT has a dual input, we'll want at least two identical parallel strings (or multiples of 2) so the power is balanced on each input.
So, let's run through an example!

Let say you take a shine to Panasonic's 330W HIT panels. So you google to find the datasheet and look up a few things:

Voc = 71V
Isc = 6.08
Temperature Coefficient (Voc) = -0.17C

You want 5 kw and the panels are 335W, so divide it out (5000/335) and that's 14.92 panels. They don't sell fractional panels, so you could round up to 15.
But alas! Your want to have an equal number of panels on each string, so let's bump it up to 16!

So, 16 panels/2 strings = 8 panels in series per string. Each panel has 71V, so with 8 panels that's 8x72= 576V -- OOPS! 576V is way over the 145V maximum of the MPPT....That's not going to work!

The voltage on the Panasonic's is fairly high; what would happen if you used something with a lower voltage like the LG Neon2's? Here's that datasheet:

Voc = 41V
Isc = 9.8
Temperature Coefficient (Voc) = -0.27C

So two strings of 8 panels would give you 328V and 9.8 amps each. Nope... Well, how about 4 parallel strings of 4 panels? Nope each string is 164V. Still too high! Could you make it work with two strings of 8 panels in parallel? That would be 34V and 78.4 amps... that's just under the limit. It would work, but how about an even lower voltage?

How about those AltE 150W panels? To get 5000W we'd need 30 panels (4.5kW)! 30 panels is nearly twice 16, that's a lot more racking and wire so it's gonna cost more!

Voc = 22.32V
Isc = 8.86
Temperature Coefficient (Voc) = -0.32%/C

With 30 panels we could do 6 parallel strings of 5 panels in series, 5x22.32 = 112V! Whew, made it! 112 < 142! Sweet, so you'd put 3 strings on the first MPPT input and 3 on the other.

But what about the amps? Each MPPT can take 80 amps, and string is 8.86 amps, so when 3 are combined it would be under 27 amps...no problem there!

Or is there?

Temperature Correction
What's the coldest on record it's been there? Your county will have a magic number for this and you'll have to meet it in order to get your system approved (There's a similar "hot" number for when you get to calculating the wiring).

You see, the colder it gets the more power the panel produces. If you're in Colorado it might be -40°C. So, that's a temperature delta from STC of 65°C (25°C - (-40°C)), Which we multiply by the Temperature Coefficient, so 65°C x .32=20.8%

(100% + 20.8%) x 22.32 Voc ~= 27 Voct, and with 5 panels in series 27 x 5 = 135v Whew! Made it! Less than 142V on the coldest possible day of the year!

There's also a temperature coefficient for current, but it works the same way.

Hopefully this was both entertaining and informative for how you can figure out your panel configuration based on your MPPT settings, the panels you pick, and local conditions.
So with this beautiful example we can design any system with suitable panels and mppt inverter. I guess the same procedure is for mppt solar chargers?

Also (correct me if i am wrong)we can use Panasonic 330 w if we wire 2s4p in every mppt because 2*71 = 142 v < 145 v and 4*6.08 = 24.32A << 80 A (suppose we dont calculate temperature co efficient)
 

svetz

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...I guess the same procedure is for mppt solar chargers?
Yes
Also (correct me if i am wrong)we can use Panasonic 330 w if we wire 2s4p in every mppt because 2*71 = 142 v < 145 v and 4*6.08 = 24.32A << 80 A (suppose we dont calculate temperature co efficient)
Looks like you've got it! So, theoretically yes, but in practice probably not due that pesky temperature coefficient.
 
Last edited:

Supervstech

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Also (correct me if i am wrong)we can use Panasonic 330 w if we wire 2s4p in every mppt because 2*71 = 142 v < 145 v and 4*6.08 = 24.32A << 80 A (suppose we dont calculate temperature co efficient)
Also (correct me if i am wrong)we can use Panasonic 330 w if we wire 2s4p in every mppt because 2*71 = 142 v < 145 v and 4*6.08 = 24.32A << 80 A (suppose we dont calculate temperature co efficient)
Unless you live in the tropics... ignoring the temp coefficient is unwise... mppt may handle an overvoltage, but it may not.
 

gzoro

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Well I think that temperature correction factor plays role only if we are close to the charge controller maximum voltage? For minimum temperature -10 and maximum about 70 degrees I made two different calculations .First with temp correction factor and the second without it. Both 2 calculations were below maximum charge controller range.

Any idea am I correct?
 

svetz

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Well I think that temperature correction factor plays role only if we are close to the charge controller maximum voltage?...
Always safest to do the calculations so no guessing is required.

Any idea am I correct?
Can't confirm the math without seeing the math (assuming changes from #15), but based on the math in #15 it does sound like you have the gist of it.
 
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