Just ordered my first system: Now to the math

[skoobus]

Let's take RNT's 20% increase and plug that in. 1440W X 120% = 1728W, still within the specs of your unit.

About the browning, that's what real-world measurement is for. Put a 24V load on your panel and measure it's output, instead of guessing about it.

Thanks for the help! I'll definitely be testing once I get them

 

[skoobus]

Yup, you're pretty much tapped at 3s/2p. Fortunately even with the 20% cold boost you're still (just) under the limit on your voltage so you'll be good.

A second charging setup with the spare panels is easy and hey, free power! :D Better to be safe than smelling magic smoke!

I agree! safety first and K.I.S.S.

think I finally have this step down thanks everyone!

 

[jamiko]

Hmm. Any reliable way I could check the unit to see it is legit? According to growatt's website their manufacturing plant is in Huizhou, China so there's that. As far as shipping yes, I believe its coming from an amazon warehouse. Due to your warning though maybe I should get in contact with growatt when I get it to make sure I'm covered (assuming they have any kind of warranty program)

Once you get the unit, you could contact Growatt with the serial number to verify the unit and warranty. Make sure the serial number is valid and nobody else has registered it. I think Growatt has a two year warranty but don't quote me on that. Did the Amazon listing mention a warranty? I'm not sure how Growatt handles warranty and support work. Working with a vendor in the US is easier than working with a vendor in China.

 

[Rednecktek]

Aaannnd much like my Fisher Price My First Solar Harbor Freight kit, you've just paid for your education. :D

It happens to all of us at one time or another.

 

[skoobus]

Once you get the unit, you could contact Growatt with the serial number to verify the unit and warranty. Make sure the serial number is valid and nobody else has registered it. I think Growatt has a two year warranty but don't quote me on that. Did the Amazon listing mention a warranty? I'm not sure how Growatt handles warranty and support work. Working with a vendor in the US is easier than working with a vendor in China.

Will do, thanks a million for the heads up

 

[rmaddy]

I haven't looked at your numbers, just the replies that state you can only use 6 of the 8 panels.

One possible solution is to use a 2nd MPPT charge controller for the other two panels. And by 2nd MPPT SCC I mean just a stand-alone SCC, not another all-in-one.

 

[wattmatters]

So, another alternative is to wire all 8 panels, in a 2S4P configuration. What you are going to see in the real-world is....

a dead charge controller when the Voc of the 4S strings on a cold sunny morning goes to 165-170V.

Keep strings to 3S max. Last thing we want is for the OP to compound their error. Better to have a system that is working than dead.

Keep the other two panels for a separate project. Or as spares, or flip 'em for a few bucks.

The OP is learning from their understandable mistake - that's what doing a first project is about.

Working out a PV array configuration is a combination of a whole range of factors beyond the wattage ratings. I recall turning my brain inside out trying to work everything out with my off-grid project, when it dawned on me that a different panel spec was going make life a whole lot easier.

 

[MichaelK]

a dead charge controller when the Voc of the 4S strings on a cold sunny morning goes to 165-170V.

Keep strings to 3S max. Last thing we want is for the OP to compound their error. Better to have a system that is working than dead.

Is there some confusion here? 2S would be ~60VDC.

 

[wattmatters]

Is there some confusion here? 2S would be ~60VDC.

Yes, apologies, my bad. I thought you had written 4S2P, not 2S4P.

2S4P is definitely an option to use all eight panels and remain within spec. Just more of a PITA to combine four strings. More connectors and wires but in the grand scheme if eight panels fit on the roof then better to have eight than six!

 

[jamiko]

Just make sure the wiring and components can handle the current. A 2S4P could be ~33-36 at test temperature correct?

I thought the panel info was:
Short circuit current (ISC): 8.78 A
Max power current: 8.31 A

 

[wattmatters]

Just make sure the wiring and components can handle the current.

Yes. Standard solar panel MC4 connectors are rated to 30A, which is why connecting a 4P arrangement for 8.x amp panels means at least two sets of wires to be combined at the charge controller. With 3P you could combine them with a triple MC4 connector at the panels and run one cable to the charge controller.

 

[nativerobin]

With your inverter only handling 30-115 volts i'd be afraid to wiring up more than two panels in a circuit, and then with them in parallel back to the inverter. If your panels aren't close to your inverter pay attention to the wire gauge you use. I Run my inverter off a solar array 192 feet from the house and my wiring size cost 500$ more than planned. If you have a higher voltage system you can use a smaller gage wire, which is cheaper.

 

[TJ King]

I guess the way to think about it, and it's called "overpaneling" is that you can't use more than 6 panels At A Time without going over your specs. So yes, like MisterSandals said, if you have a string facing south and a string facing east and a string facing west, only 1 string would be in full sun at a time.

Now, depending on where you're actually going to be sticking the panels, I'd say stick 2 strings (3s/2p) facing south for maximum performance, and then a string (3s/1p) facing east for early morning sun or west so you get more charging later in the day, whichever has the lower horizon.

And if you can get 1 more panel it will make the math easier.

Im a noob here, so Im curious about your remarks about "Overpaneling". Taking for example a 3s3p setup of these panels, I was under the apparently mistaken impression that you could use 9 panels in this setup. Why is 6 panels at a time the maximum? I was under the impression that based on his specs, 9 panels would produce, ideally 2250w and that the concept of "overpaneling" was that regardless whether some are tilted east west etc, that even if flat, they are allowed to exceed the 2000w number of his charge controller, but the Growatt would just ignore anything over 2000w. And that the benefit of the 9 panel 2250w array is that with more surface area, in suboptimal sun conditions, you would still produce more watts than you otherwise would if you only had a 2000w array. Am I misunderstanding? The growatt has a hard limit on volts of 145v leading to damage if exceeded, so if you have the 3s3p setup, you will be well below that at 111.9v. Amps also, well below specs. Which specs are limiting a person to 6 panels only? Is the math I'm thinking about applied to your remark, add one more panel and math gets easier? Such as add 1 to 8 and use 9?

 

[Rednecktek]

The idea is that for the health of the controller you don't want to do a 3s/2p pumping in 112v and then strap on a 2s set only pumping 90volts. Having voltages that far off from each other and going into the same MPPT causes "Bad Things" to happen as the voltages try to equalize out. Think about what would happen if you tried to plug a 12v battery into a 120v wall outlet. Magic smoke, fireworks, angry wife, you know the drill.

If you're only using 8 panels and connecting them up in 3s strings, then the last two panels need to go to another controller so the voltages don't try to burn each other out. 2 controllers connected won't fight each other since both controllers are working with different input numbers, but output is the same.

Yes, your concept of overpaneling is correct, the theory being that on paper if every panel was running at 100% spec you would fry the system, but in reality we don't have 3 suns that could hit panels from all the angles at the same time and as long as you're not exceeding your VoC or amperage at any one time you're still good.

Also when calculating your VoC, add 20% for stupid cold weather efficiency boost to your calculations. Panels work better cold than hot and it can potentially push you right over the edge into magic smoke territory.

Does that make sense to anyone but me?

 

[TJ King]

The idea is that for the health of the controller you don't want to do a 3s/2p pumping in 112v and then strap on a 2s set only pumping 90volts. Having voltages that far off from each other and going into the same MPPT causes "Bad Things" to happen as the voltages try to equalize out. Think about what would happen if you tried to plug a 12v battery into a 120v wall outlet. Magic smoke, fireworks, angry wife, you know the drill.

If you're only using 8 panels and connecting them up in 3s strings, then the last two panels need to go to another controller so the voltages don't try to burn each other out. 2 controllers connected won't fight each other since both controllers are working with different input numbers, but output is the same.

Yes, your concept of overpaneling is correct, the theory being that on paper if every panel was running at 100% spec you would fry the system, but in reality we don't have 3 suns that could hit panels from all the angles at the same time and as long as you're not exceeding your VoC or amperage at any one time you're still good.

Also when calculating your VoC, add 20% for stupid cold weather efficiency boost to your calculations. Panels work better cold than hot and it can potentially push you right over the edge into magic smoke territory.

Does that make sense to anyone but me?

Sorry again, as a noob, Im either not following you or Ive been wrong all along, which is very often the case. I was not aware that using different arrays of different voltages would automatically set your system on fire and destroy your controller. I was under the impression that disparate amps and/or voltages, that although a bad strategy, will deliver which ever is the lowest common denominator which is clearly suboptimal, but about the instant explosions? I was not aware or misunderstood the basics.

Im not sure which part of the "Overpaneling" I was correct about, since I was referring to a 3s/3p (250x9=2250) setup, which according to the above specs would result in a total voltage to the controller of 112v which is well below the 145v Hard Limit of the controller. And taking in to account your "Add 20%" maximum allowance for cold weather to the 112v still keeps you well below the 145v, so again, I'm not sure how this too causes smoke and fires.

The scenario for overpaneling I posited was laying the 9 panels flat so no need for 3 suns, allowing all 9 panels so as you say "every panel was running at 100%" (ie 2250w) and I don't understand how that too would "fry" my system. I get that 145v is a hard limit, which by exceeding that you may "fry" your system, but hitting 2250w, but staying safely below those other hard limits (145v) I had previously understood to be safe and the actual definition of "overpaneling". Sorry if Im being slow here, but this has got me rethinking several upgrades I had been thinking about for fear I'll burn the house down now. Thanks for offering your advice. I'll keep trying to research this

 

[wattmatters]

Sorry again, as a noob, Im either not following you or Ive been wrong all along, which is very often the case. I was not aware that using different arrays of different voltages would automatically set your system on fire and destroy your controller.

They won't and your basic understanding is correct. That said it's still a good idea not to have panels with significantly different specs combined.

There's a new info guide regarding over paneling:

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

diysolarforum.com

 

[TJ King]

Wow, what a great pdf explainer for overpaneling. I appreciate your reassurances. I'm not an electrician and I was about to cancel all of my projects for fear I was going to injure my family or worse. I'm printing this document and putting it in my binder. Thank you

They won't and your basic understanding is correct. That said it's still a good idea not to have panels with significantly different specs combined.

There's a new info guide regarding over paneling:

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

diysolarforum.com

 

[TJ King]

Wow, what a great pdf explainer for overpaneling. I appreciate your reassurances. I'm not an electrician and I was about to cancel all of my projects for fear I was going to injure my family or worse. I'm printing this document and putting it in my binder. Thank you

They won't and your basic understanding is correct. That said it's still a good idea not to have panels with significantly different specs combined.

There's a new info guide regarding over paneling:

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

diysolarforum.com

 

[12VoltInstalls]

alternative is to wire all 8 panels, in a 2S4P configuration. What you are going to see in the real-world is....
250W X 8 panels X 0.9FF X 0.80AF= 1440W. Assuming your 24V battery will charge at 25V, that works out to be 1440W/25V = 57.6A. That's well within the 80A maximum for charging amps.

That seems like a good layout imho