New all in one, small off grid cabin.

[KolobSolar]

Crosspost from all-in-one
I have the Growatt SPF 3000TL LVM-24P which I just purchased. I’m strictly off-grid and feed it with 4x100w Renogy panels. I store in 4 x 6v400ah (24v/400ah) Centennial AGM L16’s in series. I have a battery balancer on the 4 cells. My non-occupied load is 55 watts 120v AC constant rate. Starlink and security cams.

It’s monsoon season and afternoons are cloudy. There is theoretically 14 hrs of sun per day at my latitude right now. Batteries go to 25 % SOC overnight and struggle to get to 100% it’s clouds.

I’m planning to buy 3 x 450w panels. Do I need even more storage? My occupied loads only top out 170w, not continuous.

Any help would be appreciated.

 

[MattiFin]

You need more battery capacity if you discharge down to 25% regularly. Lead-acid lifetime suffers from deep dischage.

 

[pollenface]

I think there are not enough hours of GOOD sun to get your batteries fully charged. Adding 3x450w panels will definitely see an improvement. It should be enough battery capacity, but if they are constantly getting down to 25% and not getting fully charged they are probably already going down hill.

55w/0.85 = 64.7w at batteries (divide AC load by 0.85 to account for inverter inefficency - heat loss)
64.7*24hrs = 1552.8wh

 

[MichaelK]

I would say definately it's the solar that's the biggest problem. Let's look at the numbers. First, when you say the "non-occupied load" is 55 watts do you mean this is what the Growatt is consuming just being left on? 55W X 24hr = 1320Wh. Assuming your router and cams consume another 100W, that's (100W X 24hr = 2400W) + 1320W = 3720Wh, or call it 3.7kWh.

No way that 400W of panels is going to make you 3.7kWh of power. Forget about having 14 hours of daylight. You need to be focusing your attention more on your area's sunhours (sh) per day, which I'll suggest run from 2.5sh to 3.0sh in December, and 5.0sh to 6.0sh in June.

Even assuming you get a lavish 6sh right now, that's only 400W X 6sh = 2400Wh per day. So, what's happening is that your batteries are not really reaching full charge each and every day, and total charge is spiraling downwards on marginal days when the monsoon clouds reduce capacity.

Assuming your AGMs want a charge at 1/8th of C, that's (400Ah/8) X 25V charging X 1.175Fudgefactor = 1469W. Just call that 1500W.

Your idea of adding the additional 450W panels is a good one, but what you must be careful about is matching string voltages. Your four 100W panels in series will be putting out ~72Vmp, and maybe 88Voc. You need to check this. What is the voltage specs of these new 450W panels? String voltages will need to match within 5% if you want to mate them in parallel strings.

Let me take a guess and say the new panels run at ~36Vmp. If that's the case, you could wire two of them in series to get 72Vmp for the two 450s. You can't match the voltages if you have three of those. What you could do instead is buy four panels instead of three to create three strings, one 4S and two 2S. That would bring your total wattage to 400W +900W + 900W = 2200W total. (2200W/25V) charging X 85% fudgefactor =75A, so can your model of Growatt handle 75A?

Get those specifications for us, and we can help you re-design your system to make if far more optimal.

 

[MattiFin]

Your idea of adding the additional 450W panels is a good one, but what you must be careful about is matching string voltages. Your four 100W panels in series will be putting out ~72Vmp, and maybe 88Voc. You need to check this. What is the voltage specs of these new 450W panels? String voltages will need to match within 5% if you want to mate them in parallel strings.

you can also connect wildy varying strings together but you lose some of the theoretical maximum output.
combined voltage is voltage of the smallest string voltage and combined current is sum of all strings. 20A 80v string and 5A 120v string parallel makes 25A and 80volts
(Albeit some mppt controllers get confused ang get stuck to local maximum power)

 

[KolobSolar]

I think there are not enough hours of GOOD sun to get your batteries fully charged. Adding 3x450w panels will definitely see an improvement. It should be enough battery capacity, but if they are constantly getting down to 25% and not getting fully charged they are probably already going down hill.

55w/0.85 = 64.7w at batteries (divide AC load by 0.85 to account for inverter inefficency - heat loss)
64.7*24hrs = 1552.8wh

Batteries are only weeks old but I know I’m killing them with this depth of discharge.

 

[KolobSolar]

I would say definately it's the solar that's the biggest problem. Let's look at the numbers. First, when you say the "non-occupied load" is 55 watts do you mean this is what the Growatt is consuming just being left on? 55W X 24hr = 1320Wh. Assuming your router and cams consume another 100W, that's (100W X 24hr = 2400W) + 1320W = 3720Wh, or call it 3.7kWh.

No way that 400W of panels is going to make you 3.7kWh of power. Forget about having 14 hours of daylight. You need to be focusing your attention more on your area's sunhours (sh) per day, which I'll suggest run from 2.5sh to 3.0sh in December, and 5.0sh to 6.0sh in June.

Even assuming you get a lavish 6sh right now, that's only 400W X 6sh = 2400Wh per day. So, what's happening is that your batteries are not really reaching full charge each and every day, and total charge is spiraling downwards on marginal days when the monsoon clouds reduce capacity.

Assuming your AGMs want a charge at 1/8th of C, that's (400Ah/8) X 25V charging X 1.175Fudgefactor = 1469W. Just call that 1500W.

Your idea of adding the additional 450W panels is a good one, but what you must be careful about is matching string voltages. Your four 100W panels in series will be putting out ~72Vmp, and maybe 88Voc. You need to check this. What is the voltage specs of these new 450W panels? String voltages will need to match within 5% if you want to mate them in parallel strings.

Let me take a guess and say the new panels run at ~36Vmp. If that's the case, you could wire two of them in series to get 72Vmp for the two 450s. You can't match the voltages if you have three of those. What you could do instead is buy four panels instead of three to create three strings, one 4S and two 2S. That would bring your total wattage to 400W +900W + 900W = 2200W total. (2200W/25V) charging X 85% fudgefactor =75A, so can your model of Growatt handle 75A?

Get those specifications for us, and we can help you re-design your system to make if far more optimal.

The idle on the Growatt is < 50w, my usage is another 60w continuous. My thoughts on panels is to replace the 4x100’s rather than add to them. I use a pole top mount that would be tricky to carry 8 panels. I’ve attached the panel specs, AIO spec, and battery specs.

I’m convinced I need 4, not 3 and I’m fine with that.

 

[KolobSolar]

I would say definately it's the solar that's the biggest problem. Let's look at the numbers. First, when you say the "non-occupied load" is 55 watts do you mean this is what the Growatt is consuming just being left on? 55W X 24hr = 1320Wh. Assuming your router and cams consume another 100W, that's (100W X 24hr = 2400W) + 1320W = 3720Wh, or call it 3.7kWh.

No way that 400W of panels is going to make you 3.7kWh of power. Forget about having 14 hours of daylight. You need to be focusing your attention more on your area's sunhours (sh) per day, which I'll suggest run from 2.5sh to 3.0sh in December, and 5.0sh to 6.0sh in June.

Even assuming you get a lavish 6sh right now, that's only 400W X 6sh = 2400Wh per day. So, what's happening is that your batteries are not really reaching full charge each and every day, and total charge is spiraling downwards on marginal days when the monsoon clouds reduce capacity.

Assuming your AGMs want a charge at 1/8th of C, that's (400Ah/8) X 25V charging X 1.175Fudgefactor = 1469W. Just call that 1500W.

Your idea of adding the additional 450W panels is a good one, but what you must be careful about is matching string voltages. Your four 100W panels in series will be putting out ~72Vmp, and maybe 88Voc. You need to check this. What is the voltage specs of these new 450W panels? String voltages will need to match within 5% if you want to mate them in parallel strings.

Let me take a guess and say the new panels run at ~36Vmp. If that's the case, you could wire two of them in series to get 72Vmp for the two 450s. You can't match the voltages if you have three of those. What you could do instead is buy four panels instead of three to create three strings, one 4S and two 2S. That would bring your total wattage to 400W +900W + 900W = 2200W total. (2200W/25V) charging X 85% fudgefactor =75A, so can your model of Growatt handle 75A?

Get those specifications for us, and we can help you re-design your system to make if far more optimal.

Peak sun hours are a low of 4.8 and high of 7.2 through the year.

 

[MichaelK]

OK, looking at the screen shot of your panels, the number appear to be totally wrong. I searched for the 450W model and found what appear to be accurate specs. Compare below with what you see above.

So, let's do a design with the real numbers. The optimal MPPT range of the Growatt's controller is 30V to 115V, with an NEVER EXCEED voltage of 145V. That means you CANNOT wire three of those panels in series. The Voc is 50.1Voc, so already three would exceed 145V and fry the circuitry.

You can though get four, and wire two parallel strings of two panels in series. We write that in shorthand as 2S2P. With four of those panels, you can expect (450W X 4 panels)/25V = 72Amps. Your controller's maximum is 80, so that is also OK. In the real-world, I always like to include a 85% derating I call the fudgefactor. 72A X 85% = 61.2A, which is what I predict you will actually see on a day to day basis. Since the max charging rate the battery manufacturer states is 120A, your charging amperage though fairly high is in the safe zone.

With that much solar, I'd expect you to be able to make 0.9kWh even on a rainy day. You could add more solar for marginal cloudy days, but you have to be careful about exceeding the amp limit of 80A. If you wanted to go with 6 panels, wired 2S3P, you could do it, but you'd have to point one string Southeast, another South, and the third Southwest to reduce the noon peak. You'll dampen the noon peak, but broaden the power curve over the course of the day. But, you'll have another 50% power on those cloudy, marginal days.

 

[pollenface]

You could add more solar for marginal cloudy days, but you have to be careful about exceeding the amp limit of 80A.

You don't really have to worry, some reasonable overpanelling is common practice with mppts.

It's the maximum charge current, not a pv limit. It's generally deemed as acceptable to exceed this by 1.3x to 1.5x.

 

[MichaelK]

You don't really have to worry, some reasonable overpanelling is common practice with mppts.

It's the maximum charge current, not a pv limit. It's generally deemed as acceptable to exceed this by 1.3x to 1.5x.

I've very up on overpaneling. My own system is overpaneled by more than 2000W already, and plan on adding another 2000W later this summer.

 

[KolobSolar]

OK, looking at the screen shot of your panels, the number appear to be totally wrong. I searched for the 450W model and found what appear to be accurate specs. Compare below with what you see above.
View attachment 104978
So, let's do a design with the real numbers. The optimal MPPT range of the Growatt's controller is 30V to 115V, with an NEVER EXCEED voltage of 145V. That means you CANNOT wire three of those panels in series. The Voc is 50.1Voc, so already three would exceed 145V and fry the circuitry.

You can though get four, and wire two parallel strings of two panels in series. We write that in shorthand as 2S2P. With four of those panels, you can expect (450W X 4 panels)/25V = 72Amps. Your controller's maximum is 80, so that is also OK. In the real-world, I always like to include a 85% derating I call the fudgefactor. 72A X 85% = 61.2A, which is what I predict you will actually see on a day to day basis. Since the max charging rate the battery manufacturer states is 120A, your charging amperage though fairly high is in the safe zone.

With that much solar, I'd expect you to be able to make 0.9kWh even on a rainy day. You could add more solar for marginal cloudy days, but you have to be careful about exceeding the amp limit of 80A. If you wanted to go with 6 panels, wired 2S3P, you could do it, but you'd have to point one string Southeast, another South, and the third Southwest to reduce the noon peak. You'll dampen the noon peak, but broaden the power curve over the course of the day. But, you'll have another 50% power on those cloudy, marginal days.

Where does the 25V come from in the explanation above?
I haven’t pulled the trigger yet on panels and now I’m thinking 6 of these 2S3P or 3S2P

Thoughts

 

[MichaelK]

Where does the 25V come from in the explanation above?
I haven’t pulled the trigger yet on panels and now I’m thinking 6 of these 2S3P or 3S2P

That's very simple. A 24V battery doesn't charge at 24V. Depending on depth of discharge, and also battery chemistry, you are likely to see charging start at 25V. It will go up though as the state of charge also goes up.

Typically controllers are pouring in the most amps in the bulk phase, so it's useful to determine the maximal number of amps going in at the lowest voltage. What you will typically see though is that as the state of charge goes up, voltage also goes up, but the amperage goes down. With lead-acid batteries, as they approach full charge, just a fraction of the starting amps is going in at the finish.

 

[iamrich]

Crosspost from all-in-one
I have the Growatt SPF 3000TL LVM-24P My non-occupied load is 55 watts 120v AC constant rate. Starlink and security cams.

Does the 55wh include the Growatt idle usage? My LV2424 burns about 50wh by itself.

 

[KolobSolar]

Does the 55wh include the Growatt idle usage? My LV2424 burns about 50wh by itself.

No it doesn’t. The burn rate on the unit itself is about 50 watts

 

[iamrich]

No it doesn’t. The burn rate on the unit itself is about 50 watts

Yeah, that is what I figured. So, you are using 1.2kwh just sitting idle. Still, if I am doing the math correctly, you have about 5kwh of usable storage and roughly half that in usage. I would panel the heck out of the Growatt and face the strings in different directions.

I don't know what your situation is as far as space to mount panels is, but the sweet spot for this class of inverter (at least in my opinion) is used 250w 60 cell panels. Buy 9 of them, wire them 3S3P and face them SE, S, SW to lengthen your solar day. My setup routinely pushes 50a in the batteries and it is sitting flat on roof pointed more east than south. Even in crappy weather you should be able to produce a little power and assuming the sun comes out at some point you will have at least one array pointed in the correct direction.

 

[KolobSolar]

Yeah, that is what I figured. So, you are using 1.2kwh just sitting idle. Still, if I am doing the math correctly, you have about 5kwh of usable storage and roughly half that in usage. I would panel the heck out of the Growatt and face the strings in different directions.

I don't know what your situation is as far as space to mount panels is, but the sweet spot for this class of inverter (at least in my opinion) is used 250w 60 cell panels. Buy 9 of them, wire them 3S3P and face them SE, S, SW to lengthen your solar day. My setup routinely pushes 50a in the batteries and it is sitting flat on roof pointed more east than south. Even in crappy weather you should be able to produce a little power and assuming the sun comes out at some point you will have at least one array pointed in the correct direction.

Thank you, I currently have a pole top mount. Sounds like my best hope would be a custom fabrication if I wanted to use it as you described.