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Growatt 24V SPF 3000TL LVM – 3kW 120Vac Stackable Inverter 2kW MPPT Charge Controller Split Phase

DouglasPHill

DouglasPHill

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May 24, 2021
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I just order 4 255W solar panels. I know I can hook 2 together serially and it should work fine with this Growatt.
Can I add a third? I have room on the top of my bus for 3. The panel specs are: (Thank you)
Canadian Solar 255W​
Rated Power: 255W​
Open circuit voltage (VOC): 37.4 V​
Max power voltage (VMP): 30.2 V​
Short circuit current (ISC): 9.0 A​
Max power current: 8.43 A​
Power Tolerance 0/+3%​
Maximum system voltage: 600V (UL)​
Fuse Rating: 15 A​
IP65 Junction Box w/standard solar connectors​
 
DouglasPHill said:
I just order 4 255W solar panels. I know I can hook 2 together serially and it should work fine with this Growatt.
Can I add a third? I have room on the top of my bus for 3. The panel specs are: (Thank you)
Canadian Solar 255W​
Rated Power: 255W​
Open circuit voltage (VOC): 37.4 V​
Max power voltage (VMP): 30.2 V​
Short circuit current (ISC): 9.0 A​
Max power current: 8.43 A​
Power Tolerance 0/+3%​
Maximum system voltage: 600V (UL)​
Fuse Rating: 15 A​
IP65 Junction Box w/standard solar connectors​
Click to expand...
To answer the question, we have to know the PV input specs of the Growatt. I believe the following are the specs.... but check your manual!!

1635798593961.png
The key spec is the Max PV Array Open Circuit Voltage... This defines the PV no-damage voltage limit of 145V
However, for optimal operation, the key spec is the PV Array MPPT Voltage Range of 30-115V.

The only way 3 panels can be used on a single controller is to put them in series. 3 of the panels will have a Voc of 37.4 x 3 = 112.2V (@25C). That leaves plenty of room for cold temperature Voltage rise from a damage point of view but leaves essentially no room for optimal operation in cold temperatures. So, unless you are in an area that never gets below 25C, 3 of those panels is probably not a good idea.

If you can fit them, you could do two parallel strings of 2 panels in series (4 panels total). This would keep the Voc voltage in a safe range even in cold weather.
 
Everyone, thank you for your responses. Yes I was thinking I had room for 3. So I can use two panels in serial or
3 panels in parallel. Is there an advantage either way?
 
3 in series would put the operating voltage higher than the MPPT operating range in cold weather. This would not damage the controller, but I don't know how well it will capture the energy.
 
FilterGuy said:
3 in series would put the operating voltage higher than the MPPT operating range in cold weather. This would not damage the controller, but I don't know how well it will capture the energy.
Click to expand...
The charger loading the panels will pull the voltage down into the MPPT range. Fully loaded 3 of those in series would be 91v (inside the MPPT range). Those look like 60cell panels. I have 3s2p 60 cell panels, about 1500 real watts, on one of my Growatt 3000. It seems to do well.
 
DThames said:
Fully loaded 3 of those in series would be 91v (inside the MPPT range).
Click to expand...
That is when operating at MPP at 25C. The good news is that even in very cold, the Vmp will be under the 115V of the top of the MPPT operating range. However once the batteries are full and the charger turns off, the voltage will rise toward Voc and in cold weather can go above the 115V.

DThames said:
The charger loading the panels will pull the voltage down into the MPPT range.
Click to expand...
That is the key. Just what does the controller do once the input goes above the 115V top end of it's MPPT range? If it does draw enough current to pull the voltage down to below 115, then yes, all is good. If not, then what does it do? Furthermore, if it is still operating above 115V to control the voltage and pull it back down. What does PV Array MPPT Voltage Range of 30-115V mean?

Note: This might well be dependent on how cold it is and therefore how high Voc gets. Assuming a panel beta of .3, at 0C the string Voc is 120.6 and Vmp is ~97.4. At 0F the string Voc is 126.6 and Vmp is ~ 102.2. Will the controller pull enough current to get the voltage below 115 and into the operating voltage range.
 
FilterGuy said:
That is when operating at MPP at 25C. The good news is that even in very cold, the Vmp will be under the 115V of the top of the MPPT operating range. However once the batteries are full and the charger turns off, the voltage will rise toward Voc and in cold weather can go above the 115V.


That is the key. Just what does the controller do once the input goes above the 115V top end of it's MPPT range? If it does draw enough current to pull the voltage down to below 115, then yes, all is good. If not, then what does it do? Furthermore, if it is still operating above 115V to control the voltage and pull it back down. What does PV Array MPPT Voltage Range of 30-115V mean?

Note: This might well be dependent on how cold it is and therefore how high Voc gets. Assuming a panel beta of .3, at 0C the string Voc is 120.6 and Vmp is ~97.4. At 0F the string Voc is 126.6 and Vmp is ~ 102.2. Will the controller pull enough current to get the voltage below 115 and into the operating voltage range.
Click to expand...
I don't know what that range means. Below that range, I think it will work in PWM mode or something similar.

I think mine runs unloaded at about 122v-125v. I will check it again when I get the chance, when I am home and the sun is shining. I have mine connected to the "cloud" and there are some logs. I will try to look at the logs and see if there is anything telling about the voltage it was working at.
 
Attached is a log from last Saturday. The panels were fully in the sun when I closed the breaker to the Growatt. So the charger saw and recorded open circuit (sort of) voltage of 128v. Note the voltage is pulled down and let up as the charger's load on the panels varies. Nothing odd above the 115v level.
 

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DouglasPHill said:
Everyone, thank you for your responses. Yes I was thinking I had room for 3. So I can use two panels in serial or
3 panels in parallel. Is there an advantage either way?
Click to expand...
Yes, you get by with smaller gauge PV cable in series but you will have less watts total. Also shading will affect panels more in series and not parallel. If you go parallel, 10 ga will work, but I'd opt for 8 ga to limit voltage drop. You should also install a combiner box with fuses and breakers. I used this one and it would be perfect for your use. https://www.amazon.com/gp/product/B07F5HDP2W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I did remove the MC connectors, I wire direct. The original MC4's on this box are not waterproof anyway. I purchased these glands and yes, I used 8 ga PV wire on my setup. https://www.amazon.com/gp/product/B07VT317NT/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

For the extra ports for a 4th panel, you get a few options. I have 3P strings and simply used a short piece of trimmed off PV cable in the extra port glands I installed. Or you can run PV wire to a receptacle for a portable panel, as you purchased 4 panels total, you have the option to make the extra a portable panel.
 
Zwy said:
Yes, you get by with smaller gauge PV cable in series but you will have less watts total. Also shading will affect panels more in series and not parallel. If you go parallel, 10 ga will work, but I'd opt for 8 ga to limit voltage drop. You should also install a combiner box with fuses and breakers. I used this one and it would be perfect for your use. https://www.amazon.com/gp/product/B07F5HDP2W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I did remove the MC connectors, I wire direct. The original MC4's on this box are not waterproof anyway. I purchased these glands and yes, I used 8 ga PV wire on my setup. https://www.amazon.com/gp/product/B07VT317NT/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

For the extra ports for a 4th panel, you get a few options. I have 3P strings and simply used a short piece of trimmed off PV cable in the extra port glands I installed. Or you can run PV wire to a receptacle for a portable panel, as you purchased 4 panels total, you have the option to make the extra a portable panel.
Click to expand...
What is the math behind "getting less watts total" if the panels are in series?
 
FilterGuy said:
I don't understand the 'less watts total'.
Click to expand...
I once had to commute from Inglewood CA to South Gate CA, which most of the trip was through Compton CA and was "less Watts total" for the trip. Just sayin..... Come on.... we all knew he meant "Volts" Whew you guys are rough!
 
Last edited:
The "Less Watts Total" I believe is in reference to the voltage drop you'll experience if you use smaller cable over larger cable. Less Volts * Less Amps = Less Watts Total.
 
Jerrys13 said:
Come on.... we all knew he meant "Volts"
Click to expand...
Actually, I did not. Even going back and reading it again, Volts does not make sense either, but I guess it could be less current.

"Yes, you get by with smaller gauge PV cable in series but you will have less current total. "
 
FilterGuy said:
I don't understand the 'less watts total'
Click to expand...
He is limited to 2 panels in series, 255 x 2 = 510 watts. Otherwise his VMP is too high. 3P allows an extra panel, 255 x 3 = 765 watts.

Simple math where I come from..........
 
Rednecktek said:
The "Less Watts Total" I believe is in reference to the voltage drop you'll experience if you use smaller cable over larger cable. Less Volts * Less Amps = Less Watts Total.
Click to expand...
Nope, watts.

He asked about 2S compared to 3P. Last I knew 3P would be more watts total. Some don't understand the concept however that 3 panels produce more than 2 panels. ?
 
FilterGuy said:
Actually, I did not. Even going back and reading it again, Volts does not make sense either, but I guess it could be less current.

"Yes, you get by with smaller gauge PV cable in series but you will have less current total. "
Click to expand...
Yes, current (amps) would be less in series, voltage higher. Potential watts still the same whether 2S or 2P however there will be some loss with voltage drop. The original question I responded to was 2S or 3P and what were the advantages of each.

I should have made it a little more clear in first post but with all the discussion about it, maybe it might drive the point home. 3P will net him more wattage than 2S.
 
Again everyone, thank you. My head is spinning. Please correct me here: for illustration one panel = open circuit voltage 37.4, max power voltage 30.2 (I don't know the difference) and max current 8.43amps hence 30.2x8.43amp = 255W

So if I put 2s I get 60.4 volts at 8.43 amps which is 509W (round numbers here. 3p I get 30.2 at 25amps = 763W

The controller appears ok with up to 80amps. And both the 60.4 volts and the 30.2 volts are in the voltage range.(I think)
If I use 8gauge wire I should be good to go 3P. (They dropped off the panels yesterday ) :)
 
You always use VOC which in your case is 37.4. If you went 2S you would see VOC at 74.8. 3S would be 112.2 but with low temps you could exceed the rated PV range upper limit of 115V. Could it damage the CC? Possibly, even though the max rated PV voltage is 145V.

3P is probably the best choice but there are always some cons/pros. One, CC's like to see voltage 5V above battery voltage before it begins charging. If you have full sun for 4 hours or more a day, not a problem depending on battery size and usage. Low light conditions you might see charging begin until later in the day. But if you have any panel shaded, 3P is still the best choice. With panels in series, you could see very low amps coming from PV array if one panel is shaded.

You will be fine with 8 ga wire up to about 20 feet length. 25 feet would put you just over 3% voltage drop.
 
As always time goes by before I can continue work. I know this is really stupid but I couldn't figure out how to provide 12volt dc if my batteries are 24 volts. I had assumed the growatt would have a 12volt dc output. But now I'm thinking you take the AC output out of the growatt and run that to another appliance that creates clean 12 volt dc. Is that correct? Correct that the growatt doesn't do it and need another appliance to do it?
If it is done a different way, please explain. Thank you.
 
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