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St. John USVI New Build - Solar + battery + grid

dfisher25

New Member
Joined
Jan 1, 2025
Messages
12
Location
St John, USVI
Hi Everyone,

Let me start off by apologizing if I have anything wrong here. I have done quite a bit of research on this forum and the internet but I’m still a bit lost. Part of my confusion is related to me doing a new build and most of the articles / resources I find are people adding solar/battery to an existing home. I would like to streamline the process as much as possible since all components will be new.



I’m currently constructing a 1,300-SQFT cottage in St. John USVI. The power there is very expensive and very unreliable. The first reason for me wanting to do solar plus battery is to deal with the constant outages and the second reason is to lower my power bill. The cottage will be partially used for an Air BNB and partially for personal use. For the initial set up I’m trying to stay under $10K and will add additional batteries and panels as time goes on. Please let me know if this seems like a reasonable path forward or if I’m missing something.

I have calculated my worst case daily usage at 49 kw per day. This seems really high and is mostly related to the split units, pool pump and water heater which are all biased high. Since the cottage will not be occupied full time I think things will average out around 30 kw. Also, the POCO (WAPA) no longer allows net metering so I don’t have the availability or desire to sell back.

Here is the basic thought on the design.

Grid Feed ---->Growatt SYN200-XH-US ($1,600) --->Growatt 11.4kW MIN11400TL-XH-US ($1,600)---->Conventional 100-amp main panel that feeds the home. To start, I would have 12 550-watt panels ($3,000) feeding into the inverter. When I ran the math through some calculators these panels would provide approximately 11,300 kw production per year relative to my 17,000 of worst case usage (which I hope will be more like 11,000 kw once things are dialed in). I would then have two 270 amp 12 volt LiFeP04 batteries (~$4,200) tied in.

Now what I’m looking for feed back on is if this system will do what I want it to. What I want is for the solar/batteries to be primary and for it to flip to the grid automatically if we are in a deficit. If it looks like I don’t have enough solar/battery to maintain the house, I would like to automatically switch back and forth from the grid to keep everything going. I would also like for it to automatically transfer over to solar/battery if there is a grid outage. One component question I have is regarding the Growatt SYN200-XH-US. This is a pretty expensive transfer switch, it is possible to go with a cheaper model that is suited for a generator? Another question is on my main home breaker panel, are there any solar systems that integrate this in? Or will I always have a conventional breaker box that is fed by the other components?

Once again, thank you for this resource as it has been very helpful. I started off looking at Ecoflow and the like but it seems like I can get much more bang for my buck with a DIY system. Thank you in advance and sorry for any stupid questions.
 
Two things stand out, number one you're paying for a transformer and switching thingy that you don't need with most other inverters and two, this inverter uses a 400 volt battery system which is not common and they are rather expensive.
I also think you could go with smaller higher quality pv panels that may be more resistant to weather damage.
 
First, I smiled at seeing St. John in the thread title. Many happy memories over 25+ years visiting. Most recent visit in March. Anyway…

1) I quickly ran a 6.5KW system at 15-degree tilt and got just over 11KW per year as well. Will that do all you need? Only you can determine that.
2) Consider heat pump water heater. Will save much power.
3) The pool is the big power gulper. Only so much you can likely do with the pump but hopefully others here can comment on a model that doesn’t gulp too much. I didn’t notice anything mng about a mini-split for AC but suggest that is likely needed.
4) Stay away from 12V batteries. You have no freeze worries so 48V LiFePO4 is a much better option and easily expandable as budget increases. 12V means much larger cabling, which is hard to work with and expensive.
4) Consider Victron equipment unless you have a Growatt dealer on St Thomas. It’s rock solid reliable, and I’d guess that there’s at least a marine dealer for Victron there. While I don’t usually recommend marine dealers for residential, you need a decent source for parts in the USVI as shipping can be so expensive.
5) Use PVWatts to estimate production but the big thing to consider is how much you really need. Get that right and the rest should fall into place.
 
For a pool pump you can now buy pumps that run directly from PV for a very reasonable price on Amazon with the added advantage they will run sun up to sundown, just when you need them.
I was going to do this on our soon-to-be installed pool but the pool company would only use a traditional pump so that's what I'm stuck with.....for now.
 
This is very helpful, I thought the same thing on the transfer switch but they were showing it in all diagrams which had me confused. I assume I want some say to bypass the inverter to go grid only so I guess I could I accomplish through an on-off-on breaker/switch that is just downstream of the main panel. One on would go directly to the panel and the other would be the grid feed to the inverter. Does that sound correct? Would my inverter still do the automatic switching between grid and solar/battery? I don't follow on the 400 volt battery, can you elaborate on that issue? I made the same mistake on the pool pump so I'm stuck there, it is a variable speed so I can really ramp it down which will be helpful.Thank you again for the quick response.

On the panels, this is where I'm most lost. I will need to do some more research here on what is a good panel. The market seems flooded with panels and I'm having a hard time figuring out which ones are good.
 
First, I smiled at seeing St. John in the thread title. Many happy memories over 25+ years visiting. Most recent visit in March. Anyway…

1) I quickly ran a 6.5KW system at 15-degree tilt and got just over 11KW per year as well. Will that do all you need? Only you can determine that.
2) Consider heat pump water heater. Will save much power.
3) The pool is the big power gulper. Only so much you can likely do with the pump but hopefully others here can comment on a model that doesn’t gulp too much. I didn’t notice anything mng about a mini-split for AC but suggest that is likely needed.
4) Stay away from 12V batteries. You have no freeze worries so 48V LiFePO4 is a much better option and easily expandable as budget increases. 12V means much larger cabling, which is hard to work with and expensive.
4) Consider Victron equipment unless you have a Growatt dealer on St Thomas. It’s rock solid reliable, and I’d guess that there’s at least a marine dealer for Victron there. While I don’t usually recommend marine dealers for residential, you need a decent source for parts in the USVI as shipping can be so expensive.
5) Use PVWatts to estimate production but the big thing to consider is how much you really need. Get that right and the rest should fall into place.
Great to hear about St. John, its a really special place.
1) Glad to see you came up with similar math! 11KW is definitely less than I want but it will only occupy about 1/3 of my roof. This will let me add on in the future ones I figure out my demand
2) Absolutely on the heat pump water heater, that's one of my power hogs in my calculations so I will re-run the numbers with that vs. a conventional WH.
3) The pool and mini-splits are two other hogs, not much I can do there except buy the most efficient equipment I can find and limit run time.
4) Good advice on the batteries, this is another subject I don't know much about. I would've thought I would have to increase cable size as the voltage went up.
5) Thanks for the recommendation on Victron, I will check that out.
6) Absolutely, I used PVwatts to when I ran the initial calcs!

Thanks again for the quick response. Have a great day!
 
I wouldn't mess with DC pool pumps or HPHWH's. IMNSHO it's throwing money at a localized problem, and adds needless complexity to a system you appear to need to be as seamless and low maintenance as possible.

Something around a ~12K as you have outlined that is stackable in the future is where I would go. I would budget around $20K not 10, and just use it as an off-grid / backup play. Replace any dying appliances with efficient ones, but just build out your system to handle demand. You have some pretty aggressive pricing on that Growatt. I'd get the two inverters up front. 100A of demand will be plenty for anything you might have, limit costs with battery/panels, expand as funds become available. Plan for 30-60KWH of battery, start with 15, panels as outlined.
 
I wouldn't mess with DC pool pumps or HPHWH's. IMNSHO it's throwing money at a localized problem, and adds needless complexity to a system you appear to need to be as seamless and low maintenance as possible.

Something around a ~12K as you have outlined that is stackable in the future is where I would go. I would budget around $20K not 10, and just use it as an off-grid / backup play. Replace any dying appliances with efficient ones, but just build out your system to handle demand. You have some pretty aggressive pricing on that Growatt. I'd get the two inverters up front. 100A of demand will be plenty for anything you might have, limit costs with battery/panels, expand as funds become available. Plan for 30-60KWH of battery, start with 15, panels as outlined.
Thanks for the feedback! That’s exactly what I’m looking for. What did you mean by two inverters up front? Do you mean getting two inverters to start with that are tied together?
 
If I were you I'd probably go with two 12000XP's up front. They are 2500 a pop, 50A units will take 500v@32A on each of two MPPT's per inverter, so 4 fat strings if you ever get there. This unit did not exist when I set up mine, I have the 12000's big brother at twice the cost each. The 12000's have a built-in ATS/relay for grid attachment. . . KISS. Go ahead and get them in parallel. It looks like some of the later firmware fixes a number of the parallel issues I was plagued with when the paint was still wet on all this stuff. This will give you a clear 100A+ of output capability which is what I have, and should run pretty much everything you have all at the same time. This is the demand problem, you mostly solve it up front, and get the complicated wiring and busses you need to grow all in play at the start.

Next you have the battery problem. Your going to need around 25-30KW of battery to hit that demand level. Reality is that will be rare, and the ATS will just kick you over to the grid if the inverters can't hack it, or the batteries get low. 15KW of batteries should get you north of 62A of output (100A * 50v = 15KW = 62A@240v) I would go rack but placement issues might put you in 15KW wall mount units. This will be the most expensive thing you will buy, and the most annoying to work with. Boa cables, around $4-5K for each 15KW of battery, assuming you are going with a UL listed pre-assembled 48v battery with a BMS.

Your "average"/mean usage is probably pretty low. You can get that from your bill, divide it out. Additional battery past 30KW is just buying you time with no sunshine. You might consider the size of the panels as well. 550W panels are HUGE and annoying to handle, but if they are a good fit . . . I just paid $100 for a 400W panel roughly 45x68, about as big as I want to manage. The 455's on the roof are like 45 x 80, 550's are like 48x96 or something. I'd shop that a little harder, I got a pallet of 31 400W's for about $3K all in with shipping. I'd check my daily average and figure my panels at about 50% for 6-8 hours (Winter-Summer) of output on a good day to get a rough idea of your ROI. Way too many factors to nail it down much closer until you get it installed and running.

If you have a hot tub with electric heaters and big pumps, that can create interesting but generally short lived demand. Often in the evenings. If you expect to run it at night, you might get creative with the grid switchover, just kick over when the hot tub is on. Let us know what you end up with.
 
Oh, you said around 50KWh/day, so you can probably knock your bill down 95%+ with 30KWh of batteries. Probably 65% maybe more with 15KWh, assuming you can get them charged up. I'm afraid you will likely need around 10-12 KW of panels to actually get there, but you can always tweak the settings on the inverter to take advantage of whatever you have.
 
Thank you again everyone for your help on this. After reviewing the responses it seems like I would be better off to just go all in either the EG 18KPV so I can have the full grid pass through amperage and will have the ability to add additional battery and solar panels in the future. Here is what I'm thinking for equipment:

100-Amp Fused Disconnect – Siemens GF223NR
On-Off-On switch – Eaton ‎DT223URH-N
Inverter – EG 18KPV
Battery – PowerPro 48V/280aH
Panels – Still working on size and type but I’m think about starting around 11Kw and increasing in the future.

My concern with the battery is that it is a single unit so if something fails I'm completely out of luck. Is it a better bet to go with several smaller batteries? Looking at the wiring diagram in the EG manual it seems like there wiring diagram is very complicated and the way they have the feeder tap and PV Interactive System 2 pole disconnect don't make much sense to me. Can anyone weigh in on this proposed layout of components? Also, if anyone has any other equipment suggestions it would be great to hear them!
 

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Just use their wall mount power pro battery, either the indoor or the outdoor version depending where you want to put them. In theory you should have 2 batteries to enable the 18K PV to achieve its maximum output.
 
Hi Dfisher,

I'm next door to you in STX. Electrical contractor as well. I'm on the verge of doing our first solar install and it's for my own house in STX. I just purchased the FlexBoss21, GridBoss , and Wallmount Batteries. All from EG4. Hopefully it's the right choice. Attached is the one line diagram that I'm intending on installing. The intent is to be tied to WAPA as backup and for whatever I can't produce in PV. Currently I'm backed up with an 11kW generator that runs the whole house without missing a beat. I have no idea what kind of pv production I'll be gettting with this setup but hopefully it will minimize the WAPA bill.

All are welcome to provide feedback on this setup as I'm brand new to this world.
 

Attachments

Hi Dfisher,

I'm next door to you in STX. Electrical contractor as well. I'm on the verge of doing our first solar install and it's for my own house in STX. I just purchased the FlexBoss21, GridBoss , and Wallmount Batteries. All from EG4. Hopefully it's the right choice. Attached is the one line diagram that I'm intending on installing. The intent is to be tied to WAPA as backup and for whatever I can't produce in PV. Currently I'm backed up with an 11kW generator that runs the whole house without missing a beat. I have no idea what kind of pv production I'll be gettting with this setup but hopefully it will minimize the WAPA bill.

All are welcome to provide feedback on this setup as I'm brand new to this world.
Hey there LKdiaz, good to talk to someone that is doing something similar to me. Couple questions for you, are you pulling permits from DPNR? If so, do you think they will require a fused disconnect between the grid and Gridboss? Also, with all the WAPA surge issues, do you have any concern about the Gridboss getting fried? I know sensitive electronics have a tendency to get burned up during their outages. Thanks again for the feedback!
 
Hey there LKdiaz, good to talk to someone that is doing something similar to me. Couple questions for you, are you pulling permits from DPNR? If so, do you think they will require a fused disconnect between the grid and Gridboss? Also, with all the WAPA surge issues, do you have any concern about the Gridboss getting fried? I know sensitive electronics have a tendency to get burned up during their outages. Thanks again for the feedback!
Sent you a DM.
 

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