Beginner: Planning out full Off-Grid PV system

[Shinzul]

Hey everyone - I have questions!

Basic background info:
- Building a large cabin in the woods
- Designing for being fully off-grid year-round
- Will install a large backup propane generator as a just-in-case (pays to be prepared)
- Solar panels will be ground-mounted
- Prefer the solar panels be as far as science allows from the house

I'm trying to design a system that operates year-round without needing to pull from a generator, but it's OK if it needs a few days here and there.

Note: I have already planned out my power usage using the spreadsheet provided by this forum, thank you!

I am trying to pick equipment to use. I was thinking a 48v system with:
* 2 pallets of these Hyundai 390w panels, planning to use 48 panels for a max peak of 18,720w
* 57.6 kWh of these LifePower4 batteries in 2 separate server-racks
-- this should be ~2 days of full backup power, assuming 48 hours of zero solar production

edit: I'm getting a bit hung up on which inverter(s) to use. There a growatt 12kw split-phase off-grid inverter that seems great, but I would need 2 of these, plus the max PV power on the spec sheet says 7,000w. Doesn't this mean that with 2 of them, it could only do a total of 14,000w incoming from the panels, which wouldn't be enough to charge the batteries and power the house during the winter?

So, the 12kw growatt doesn't stack, so that's not an option for my setup.

edit: I did some more research, plus I went ahead and called Signature Solar and spoke with the Sales Manager (Brent) who was SUPER helpful! Adding the answers that I found inline below, in case they're useful for anyone else later on.

I could use 4 of these stackable growatt 5,000 ES inverters instead. Questions:
1) How does the max solar charge current work when these are in parallel? Does it stack the max currents? e.g.: if all 4 were paralleled, then would the max charge current be 400 amps? That would fit with my max charge current for a 1.5d recharge period.
A) Yes, these would need a transformer, and should do 2 inverters -> 1 transformer
2) Would an auto-transformer be needed in this setup?
A) yes
3) The panel configuration I was thinking is 4s12p. How do the charge controllers send enough current into the batteries when the total current coming in from the panels at max power would only be 115.92amps? (12 parallel x 9.66 amps per panel)
A) actually, a better configuration would be two of each: 7s2p, and 6s2p, total of 4 arrays
4) How do I figure out or calculate how far away the solar panels can be from the house?
A) apparently with PV wire anything <200 feet is a safe bet
4a) With the 4s12p (am I notating that correctly, or is it parallel count/series?) configuration that I was thinking, this would still only be a single positive wire and single negative wire coming into the house, right?
A) yes, 1 pair (+/-) home run PV wire per array, but note that for the inverter PV input, you need 1 array per MPPT input (many large inverters have >1 MPPT input on them)
4b) If so, would these individual lines be split into 4 somehow and sent into each inverter? ?
A) no, have 4 sets (+/-) of PV wires for 4 different panel arrays
5) I couldn't find anything on the spec sheet for the inverters on this, but if the incoming wattage from the panels is higher (due to being cold, etc), can the inverters automatically clip the incoming power without being damaged?
A) (only 90% certain on this one) based on a thread here with an over-paneling whitepaper, I don't think the growatt inverters support clipping, so you need to stay below the 6kw PV input per inverter

I would really appreciate any input or help you all have about this! I want to make sure I get everything 100% planned out exactly before I go purchase ~$32,000 worth of solar equipment... ??

I don't know if it's worth mentioning, but I am planning to buy everything I can from signature solar. Any discount codes I can use? ?

Thanks so much!

 

[Shinzul]

Any thoughts?

 

[DThames]

It is difficult for most people to spend the time to study your plan in detail, so you might not get any comprehensive comments. I would say that with 1.5 days to recharge while loaded, you should expect to use your generator some. A few cloudy days in a row and your battery will get behind the curve pretty quickly. If we have heavy dark overcast, I see less than 5% of what I get for a normal sunny day. So it might as well be zero. The generator will allow you to operate without a huge battery and without an array that can do full charge in 0.5 days.

Note the inverter that can run without a battery seem to be better at consuming solar power more directly than the type that must have a battery. The type that must have a battery appear to power the inverter directly from the battery voltage, so solar must pass through the charger and the charger control logic to keep the battery up under load. So you are in a good track with that specific inverter model (no battery required).

 

[Shinzul]

It is difficult for most people to spend the time to study your plan in detail, so you might not get any comprehensive comments. I would say that with 1.5 days to recharge while loaded, you should expect to use your generator some. A few cloudy days in a row and your battery will get behind the curve pretty quickly. If we have heavy dark overcast, I see less than 5% of what I get for a normal sunny day. So it might as well be zero. The generator will allow you to operate without a huge battery and without an array that can do full charge in 0.5 days.

Note the inverter that can run without a battery seem to be better at consuming solar power more directly than the type that must have a battery. The type that must have a battery appear to power the inverter directly from the battery voltage, so solar must pass through the charger and the charger control logic to keep the battery up under load. So you are in a good track with that specific inverter model (no battery required).

Makes sense - I edited down my original post to the most relevant questions .

 

[DThames]

Each inverter will have one or more PV inputs with an MPPT charger per input. You need a separate array for each MPPT. All charging amps will add up.

If you only have 230v out, you would need an autotransformer to deliver 115v.

Look at this inverter by Phocos. They have three long videos on Youtube explaining its functions. I think it might have some advantages, but I have no experience with it. They also have a 230v only version that will take a lot more PV input.

 

[DThames]

With this large of a system, you want to have has high a voltage PV input as you can get, so to reduce the amps, and keep the wire sized smaller. Distance of your PV run is tied to wire size, amps, and distance. You can put it as far away as you can afford wire and power losses.

 

[Shinzul]

Awesome, thank you so much!

I've edited my original post with all the answers I've found.

 

[DThames]

Note, "but note that for the inverter PV input, you need 1 array per inverter" should be, "1 array per MPPT input". Many larger inverters have more than one MPPT unit.

 

[MichaelK]

Really where you should be starting is with a power audit, documenting here what it is that you actually want to power. Knowing your location also helps. Will you be needing electricity more for heating, or air-conditioning. Solar in Norway is going to be different from solar in Mexico.

You appear to be spending far more thought into what stuff you want instead of what you need to power. The proper way to go about this is determine what needs powering first, then determining what you need to reliably make that power. You are putting the cart before the horse.

Keep in mind that anything either adding or removing heat will require FAR more power than things like electronics, or even power tools. So, the big-ticket items in your home will be the freezers, refrigerators, air-conditioners, well-pumps, electric stoves, and electric hot water, in more or less lower to greater levels of consumption. Start with the numbers, then move on to design.

 

[OffGridInTheCity]

An overview perspective...

I operate an offgrid (with grid assist by ATS) system with 13kw PV, 108kwh battery bank and dual 12,000w inverters. I'm in Southern Oregon with a few bad months of rain/cloud per year - avg 18" rain/year.

* The system produces ~ 18,000kwh/year and due to mostly inverter losses I get 15,000kwh/year actual consumption.
* I operate with low DOD, and my numbers suggest that 13kw PV will do well with a 40kwh battery bank at 80% DOD to be able to capture/use most of the PV.
* I can operate my home pretty well for 8-9 months of the year but the 3-4 winter months are a challenge

Here's my data for the last couple of years to illustrate the issue with 100% off-grid:
I was going along thinking winter = 500- 600kwh = ~18kwh/day and then WAM, last December a heavy month of
deep clouds cut my consumable power to only 262kwh for the month on a 13kw PV array!



Here's that horrible December breakdown by day - remember this is 13kw PV array...
I didn't consume each day because the battery didn't charge up enough to trip the inverter on....

Notice Dec 23 thru Dec 28th - 6 days in a row and Dec 26 <1kwh?!? - difficult to build a battery big enough for this kind of weather gap.

Sigh....
I just don't have room to triple my array to account for this recent December shocker - so I've reluctantly started acquiring 100lb propane bottles working on a backup generator setup to charge my batteries.

 

[Shinzul]

Really where you should be starting is with a power audit, documenting here what it is that you actually want to power. Knowing your location also helps. Will you be needing electricity more for heating, or air-conditioning. Solar in Norway is going to be different from solar in Mexico.

You appear to be spending far more thought into what stuff you want instead of what you need to power. The proper way to go about this is determine what needs powering first, then determining what you need to reliably make that power. You are putting the cart before the horse.

Keep in mind that anything either adding or removing heat will require FAR more power than things like electronics, or even power tools. So, the big-ticket items in your home will be the freezers, refrigerators, air-conditioners, well-pumps, electric stoves, and electric hot water, in more or less lower to greater levels of consumption. Start with the numbers, then move on to design.

I've already planned out my power usage extensively using the awesome spreadsheet from this forum. In my original post, I had the table with the data in it, but I figured it was too verbose so I took it out.

 

[Shinzul]

An overview perspective...

I operate an offgrid (with grid assist by ATS) system with 13kw PV, 108kwh battery bank and dual 12,000w inverters. I'm in Southern Oregon with a few bad months of rain/cloud per year - avg 18" rain/year.

* The system produces ~ 18,000kwh/year and due to mostly inverter losses I get 15,000kwh/year actual consumption.
* I operate with low DOD, and my numbers suggest that 13kw PV will do well with a 40kwh battery bank at 80% DOD to be able to capture/use most of the PV.
* I can operate my home pretty well for 8-9 months of the year but the 3-4 winter months are a challenge

Here's my data for the last couple of years to illustrate the issue with 100% off-grid:
I was going along thinking winter = 500- 600kwh = ~18kwh/day and then WAM, last December a heavy month of
deep clouds cut my consumable power to only 262kwh for the month on a 13kw PV array!

View attachment 102193

Here's that horrible December breakdown by day - remember this is 13kw PV array...
I didn't consume each day because the battery didn't charge up enough to trip the inverter on....
View attachment 102195
Notice Dec 23 thru Dec 28th - 6 days in a row and Dec 26 <1kwh?!? - difficult to build a battery big enough for this kind of weather gap.

Sigh....
I just don't have room to triple my array to account for this recent December shocker - so I've reluctantly started acquiring 100lb propane bottles working on a backup generator setup to charge my batteries.

This data is fantastic - thank you so much for sharing! My location is in West Virginia, so I don't think my winters will be as cloudy in the general case as yours, but definitely over-paneling for these kinds of situations seems really important.

 

[toms]

You will find that sizing your battery at 120% max overnight useage, and your PV to recharge your battery to full approx. 75% if the year will yield the most cost effective system.

Any time you need more power get it from a generator.

 

[MichaelK]

You will find that sizing your battery at 120% max overnight useage, and your PV to recharge your battery to full approx. 75% if the year will yield the most cost effective system.

Until you get a rainy day. I've scaled my system to last me 5 days without sun, and I haven't run my generator for two years. I only run it now to run it, so the piston doesn't rust in the cylinder again.

 

[toms]

Until you get a rainy day. I've scaled my system to last me 5 days without sun, and I haven't run my generator for two years. I only run it now to run it, so the piston doesn't rust in the cylinder again.

Typical household uses 20kwh/day, comprising of 6kwh overnight, 14kwh daytime.

If you size for 5 days with no sun that is a 100kwh battery.

If you follow my guidelines you have a 10kwh battery, and run your generator for 200 hours / year.

The fuel cost over the life of the batteries is lower than the cost of the extra batteries.

Of course every system is different, but that works for most households.

Keep in mind that every off-grid system needs a generator. The generator needs to be serviced (oil/filter) every year regardless of use. All you are weighing off is fuel useage vs excess battery cost.

I bet if you give me the actual numbers of your system it’s either tiny (ie not a typical household), or it would be cheaper over a ten year period using my method.

 

[MichaelK]

I bet if you give me the actual numbers of your system it’s either tiny (ie not a typical household), or it would be cheaper over a ten year period using my method.

Well, I guess I'm not typical. My primary solar arrays are 4500W total with two accessory arrays adding another 2000W. Battery is 568Ah Rolls at 48V. I can consume 20kWh of power per day on irrigation days, running the well-pump. Routine day to day consumption is on the order of 3-4kWh. If it was a serious problem, I could get that down to 2.5kWh, keeping everything off.

My system has given me 100% for years now. No way I would ever chose your recommendations.

 

[toms]

Well, I guess I'm not typical. My primary solar arrays are 4500W total with two accessory arrays adding another 2000W. Battery is 568Ah Rolls at 48V. I can consume 20kWh of power per day on irrigation days, running the well-pump. Routine day to day consumption is on the order of 3-4kWh. If it was a serious problem, I could get that down to 2.5kWh, keeping everything off.

My system has given me 100% for years now. No way I would ever chose your recommendations.

Sure, for tiny systems the battery cost to cover multiple days use isn’t prohibitive.

My advice was aimed at typical household useage, where people are still given advice to aim for multiple days of no sun.

If you needed to use 20kwh/day every day you might have different ideas.