Just ordered my first system: Now to the math

[skoobus]

Hi all, just ordered my first solar system. I'm trying to keep it rather simple so I went with an all-in-one setup. Never done this before so I was hoping to get some of you lovely people to make sure I'm on the right track and hopefully help me avoid breaking anything once I get it all put together for the first time.

Mostly I'm trying to figure out how best to wire the panels with regards to series and parallel. I'm reading everything I can get my hands on but I'm not exactly sure what numbers to use in respect to VOC, VMP, ISC etc. I know for sure the battery bank will be 24 volts. Any help is greatly appreciated!

Here's what I got:

8 solar panels

CSUN CSUN25060M 250W Monocrystalline Solar Panel for sale online | eBay

Find many great new & used options and get the best deals for CSUN CSUN25060M 250W Monocrystalline Solar Panel at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Rated Power: 250W
Open circuit voltage (VOC): 37.3 V
Max power voltage (VMP): 30.1 V
Short circuit current (ISC): 8.78 A
Max power current: 8.31 A
Maximum system voltage: 600V (UL)

24v AIO Growatt 3000watt inverter/charger/MPPT. (here's the MPPT specs)

https://www.amazon.com/gp/product/B08P5GDN1G/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

SOLAR CHARGER​

Maximum PV array power	2000W
MPPT range@operating voltage	30VDC-115VDC
Maximum PV open circuit Volt	145VDC
Maximum solar charge current	80A
Maximum efficiency	97%
Standby power consumption	2W

6 of the ECI power 100ah batteries (in 24 volt bank)

https://www.amazon.com/gp/product/B0973LVN1X/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

 

[Rednecktek]

So the VoC is one of the big ones. When you serial up your panels, the VoC will add together. It looks like you can put 3 in series to give you a max of 112v which is in the sweet spot of your charger. If you put in a 4th panel it would end up being 149.2v and you'd let out the greasy orange smoke which would be, and forgive the technical term here, a "Bad Thing".

The higher the voltage the less sunlight your panels need to get their charge on. You'll still get a max of 8.3 amps on there, but at 112v that's 929 watts per string of panels. If you set up 6 panels in a 3s/2p setup (3 in Series and 2 sets in Parallel) that tops you out at 1858 watts which is about all your controller can use.

That's the basic answer, and there are ALWAYS the "Yeah, But..." that come into play like shading and overpaneling, but you said you're trying to keep it simple so there ya go. Do a 3s/2p setup and call it a day.

 

[skoobus]

That's the basic answer, and there are ALWAYS the "Yeah, But..." that come into play like shading and overpaneling, but you said you're trying to keep it simple so there ya go. Do a 3s/2p setup and call it a day.

Well I've got 8 panels coming in, you think I can only use 6 of them?

 

[MisterSandals]

With 1 more panel you could make a 3S3P array. You could spread your array east to west and harvest longer at a rate your SCC can handle. Something like a 3S string facing east-ish, one facing south, one facing west-ish.

 

[wattmatters]

When you serial up your panels, the VoC will add together. It looks like you can put 3 in series to give you a max of 112v which is in the sweet spot of your charger.

Yes, strings of 3 of those panels is perfect. 4 is definitely too many.

With a MPPT limit of 2000W, then I agree a 3S2P arrangement is the right choice.

Adding one more panel to go for a 3S3P array is possible as you say (oriented one string differently) but for a starter system it's generally good idea to stay with the AIO inverter's spec limits.

 

[Rednecktek]

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.

 

[skoobus]

With 1 more panel you could make a 3S3P array. You could spread your array east to west and harvest longer at a rate your SCC can handle. Something like a 3S string facing east-ish, one facing south, one facing west-ish.

Sadly my panels will be on the flat roof of my bus conversion. I also think I might be limited to 8 panels total depending on how I build the rack out.

It looks like everyone's recommendations for 3s2p is how I'll go with this, I've already bought the extra two panels so either I'll see about affording a small MPPT to add on or I'll just hang on to them just in case. Thanks everybody

 

[svetz]

You might find these self-help threads of interest:

• How much Solar Energy should I expect where I live?
• What does it mean to have solar panels in parallel and series?
• How to figure out how much Solar and inverter wattage is needed to charge a battery
• Figuring out how many panels in series and parallel based on your MPPT

 

[MisterSandals]

Its unfortunate that we see so many people buying equipment first then start doing research.

 

[Rednecktek]

If you really have the burning desire to install and use those last 2 panels, you could set them up in series and run them through a second MPPT in parallel with your big one. They won't fight each other and you'll get some power out of the extra glass up there.

Its unfortunate that we see so many people buying equipment first then start doing research.

Or we get people like me who have nothing to do but research because of their jobs. :D

 

[skoobus]

Its unfortunate that we see so many people buying equipment first then start doing research.

I agree I made a mistake! But from the specs listed my AIO said max pv 2000 watts, and that's what I based it on. They are also used panels so I doubt they will reliably get to the full 250 watts each.

I learn by experience, and can't experience much until I have it in my hands

 

[jamiko]

You bought from "WUHAN JIA XIN KANG SHUN KE JI YOU XIAN GONG SI" via Amazon. See attached screenshot. Who is that? The listing says 4-7 days, but the company is from China. China shipping is slow. The listing does say shipped from Amazon so maybe 4-7 days is correct.

I did not see the model number listed. Which model are you getting? I hear a lot about counterfeit MPP Solar units, but not much about counterfeit Growatts. Make sure you get a legit unit for safety, warranty, etc.

The Amazon listing is $859 with free shipping
Watts247.com has the 24V SPF 3000TL LVM for $760 + $40? shipping (assuming that's your model)
So you might have gotten it cheaper from possibly a more reputable seller.

 

[skoobus]

You bought from "WUHAN JIA XIN KANG SHUN KE JI YOU XIAN GONG SI" via Amazon. See attached screenshot. Who is that? The listing says 4-7 days, but the company is from China. China shipping is slow. The listing does say shipped from Amazon so maybe 4-7 days is correct.

I did not see the model number listed. Which model are you getting? I hear a lot about counterfeit MPP Solar units, but not much about counterfeit Growatts. Make sure you get a legit unit for safety, warranty, etc.

The Amazon listing is $859 with free shipping
Watts247.com has the 24V SPF 3000TL LVM for $760 + $40? shipping (assuming that's your model)
So you might have gotten it cheaper from possibly a more reputable seller.

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)

 

[robby]

Its unfortunate that we see so many people buying equipment first then start doing research.

I read the threads title and was wondering the same thing, it should have been:

"I Just finished the Math and I am now ready to order my system".
or better yet
"I just finished the Math can you double check it before I order my system"

 

[MichaelK]

One critical point here is the fact that the panels will be mounted flat on the roof, and not likely to ever be pointed directly at the sun. They will be on the hot roof all day and their voltage will drop. That means in the real world that your panels will never be at full output, whatever that might be.

Whenever I'm doing the math on a solar project, I incorporate a fudgefactor (FF) to compensate for panel inefficiencies. You can accurately determine your FF in the real-world by taking whatever panel you have, point it exactly perpendicular to the sun at noon, and measure how many real-world watts are coming off. Then you simply divide the measured Watts by the rated Watts. I've seen real-world numbers between 0.85(Renogy to 0.94(Astronergy). Let's go with 0.9 as an average.

The second component here is the angle at which the panels face the sun (AF). Assume that laying flat, the panels will never get past 80% of what they are capable of (that first FF).

So, another 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.

 

[skoobus]

I read the threads title and was wondering the same thing, it should have been:

"I Just finished the Math and I am now ready to order my system".
or better yet
"I just finished the Math can you double check it before I order my system"

I have been doing math on this for months, but sure I'm willing to collect the arrows in my back if need be lol. I actually did consider saying that as my title but I was referring to the math of going either series or parallel.. who cares anyway. Its the internet! People love to start crap I don't mind.

 

[Rednecktek]

There's still the 20% VoC boost when the panels get really cold to factor into that.

It sounds like you've got a good handle on the math, just sometimes the numbers in the real world don't divide evenly on both ends and you end up a panel short or a couple extra. :D

 

[skoobus]

One critical point here is the fact that the panels will be mounted flat on the roof, and not likely to ever be pointed directly at the sun. They will be on the hot roof all day and their voltage will drop. That means in the real world that your panels will never be at full output, whatever that might be.

Whenever I'm doing the math on a solar project, I incorporate a fudgefactor (FF) to compensate for panel inefficiencies. You can accurately determine your FF in the real-world by taking whatever panel you have, point it exactly perpendicular to the sun at noon, and measure how many real-world watts are coming off. Then you simply divide the measured Watts by the rated Watts. I've seen real-world numbers between 0.85(Renogy to 0.94(Astronergy). Let's go with 0.9 as an average.

The second component here is the angle at which the panels face the sun (AF). Assume that laying flat, the panels will never get past 80% of what they are capable of (that first FF).

So, another 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.

I have heard that laying them flat would most likely drop them to 80% or so, but I'll also be operating in winter (I live in midwest US) so if it somehow did reach its peak because of the cold or something is that a worry?

Also consider the panels are used and described as "browning" so I imagine they will have dropped in efficiency a bit

 

[MichaelK]

I have heard that laying them flat would most likely drop them to 80% or so, but I'll also be operating in winter (I live in midwest US) so if it somehow did reach its peak because of the cold or something is that a worry?

Also consider the panels are used and described as "browning" so I imagine they will have dropped in efficiency a bit

Let's take RNT's 20% increase and plug that in. 1440W X 120% = 1728W, still within the specs of your unit. Keep in mind that your panels will be coldest in early morning, when the sun is at it's most acute angle, which is going to drastically decrease output far more then temperature related increases.

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.

 

[Rednecktek]

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!