Help provide a path forward, up north woods with backup

[Hedges]

Please correct my if my line of thinking is incorrect, but I was just going to go grid-tie with backfeed due to the limited amount of hours I can collect.

Best you can do is buy grid-tie PV for $1/watt purchase price, amortizes out to $0.025/kWh over 20 years (declining further if working beyond that.) Compare to your $0.12/kWh rates if you use the watts as produced, or to $0.035 credit for backfeed (which makes oversizing not worthwhile.)

My thought is have batteries to harness overproduction during peak hours and backup when grid fails, and grid-tie with back feed for when I run out of solar due to my limited collection times. Once confirmed that things perform well expand at a later time.

My primary battery concern is to ensure that when grid goes out I can have running water and refrigeration.

Good battery backup system is much more expensive than GT PV.
Low cost hybrid inverters no so much of a premium, but don't know about lifespan.
Small refrigeration and small booster pump easy enough. Well pump is what needs multi-kW, which is why I suggest gas generator and water tank.

I have to review code yet, but yes I believe I will likely have to have UL-1741 as well. I haven't got a good grasp at how much more complicated adding a transformer for split-phase will be.

We've always had to have UL-1741 to backfeed grid.
More recently UL-1741-SA, which allows higher percentage of grid power coming from PV without destabilizing it. If you made a zero-backfeed system that likely wouldn't be needed.
Backfeed is the simplest, even of they gave zero credit.

Sunny Island was developed for 220V market, a good fit for moderate size systems there. For off-grid, a 120/240V transformer would boost U.S. 120V model that to the 240V needed for Sunny Boys. If on-grid, 6.7kW maximum draw or backfeed through relay, and all current lands on one phase of utility, may exceed 120% rule for breaker panel. A second 120/240V transformer on input could fix that. I used multiple Sunny Islands, bigger and more expensive system.

If you want to spend the money (and get federal credit while available), $5800 for 2x SI, however many Sunny Boy, and you can operate lots of stuff off-grid if ever needed. Using Lithium battery, one with compatible BMS is preferred. REC works for DIY battery.

SMA Sunny Island Off-Grid Inverter (SI6048-US-10) | eBay

SMA Sunny Island Inverter (SI6048-10).

www.ebay.com

I will try to find this insolation calculator to see if it provides more detail than the solar calculators I have used so far.

I did see that Sunny Boy has three MPPT channels and I was wondering what the best way to connect 18ish panels to them would be.

Possibly 6 panels in series for each MPPT.
First calculation is Voc of string, adjusted for record cold temperature (may be 16% increase in voltage, more or less.)

So it would be better to place each row of 6 on a channel so shade is equal on all three channels throughout the day? I could get by without optimizers?

Yes if multiple strings wired in parallel into one MPPT.
If single series string of panels per MPPT, no need for optimizers regardless.
If roof mount, RSD may be required. Not needed for ground mount.

 

[Shimmy]

I think the battery is not going to be justifiable on economic terms, so you need to shift that into the "project requirements" bucket. Be aware that an inverter that can do 7.5kW likely requires a minimum of 7.5kWh of battery, and likely closer to 10kWh. That is going to be at least $3k of your budget. Adding $4-5k for the inverter and charge controller you are left with $7k for balance of system.

Would roof mount increase your available hours?

Personally, for your situation I would just go microinverters for the solar (with them being over-paneled they will give you more steady output across a wider range of lighting conditions), and have a separate battery and inverter for the critical loads. Is it not practical to do a holding tank for water so you don't need to run the well pump (or run it on a portable generator)? It would allow you to put more money into the things that will give you value; your inverter is "too big" for such a low-use load.

 

[skorch]

Calculation of Solar Insolation | PVEducation

www.pveducation.org

Thank you looking at it now.

 

[skorch]

At those low rates, unless I am misreading you, 12 cents per kwh and 4cents per kwh on the reduced cost meter, I fail to see that you could ever save by going solar. For emergency backup or for a hobby it might be worth it though.

I assume they do not pay you for excess generated power but rather credit you against your Bill. In other words you never get a check even if you produce way more than you consume.

You are reading that correctly, that is what is making me pull the trigger on solar so hard. I have a flat charge of $33/mo. for the additional meter as well. You are correct just a credit against my bill.

 

[skorch]

Small refrigeration and small booster pump easy enough. Well pump is what needs multi-kW, which is why I suggest gas generator and water tank.

I have never considered a water tank. I'd have to look into what that would look like with maintenance and system-wise.

Sunny Island was developed for 220V market, a good fit for moderate size systems there. For off-grid, a 120/240V transformer would boost U.S. 120V model that to the 240V needed for Sunny Boys. If on-grid, 6.7kW maximum draw or backfeed through relay, and all current lands on one phase of utility, may exceed 120% rule for breaker panel. A second 120/240V transformer on input could fix that. I used multiple Sunny Islands, bigger and more expensive system

I believe you are correct, I think with a 200A service I have to look again but can go up to 10kW.

If you want to spend the money (and get federal credit while available), $5800 for 2x SI, however many Sunny Boy, and you can operate lots of stuff off-grid if ever needed.

Could you please tell me why I have seen people prefer to go with multiple small inverters instead of one larger inverter? It seems more cost efficient to purchase and install a larger one.

REC works for DIY battery

Are you referring to the same brand as the solar panels?

 

[skorch]

I think the battery is not going to be justifiable on economic terms, so you need to shift that into the "project requirements" bucket. Be aware that an inverter that can do 7.5kW likely requires a minimum of 7.5kWh of battery, and likely closer to 10kWh. That is going to be at least $3k of your budget. Adding $4-5k for the inverter and charge controller you are left with $7k for balance of system.

It is a realization that I am trying not to admit after spending so much time trying to find work-arounds. This forum was my last attempt to find something I missed. I try not to just go asking forums right away in effort to waste people's time. No battery is a lot cheaper, but it is the original reason to look into it otherwise just use a generator and be done.

Would roof mount increase your available hours?

Unfortunately it does not, just offsets the hours a bit earlier.

Personally, for your situation I would just go microinverters for the solar (with them being over-paneled they will give you more steady output across a wider range of lighting conditions), and have a separate battery and inverter for the critical loads. Is it not practical to do a holding tank for water so you don't need to run the well pump (or run it on a portable generator)? It would allow you to put more money into the things that will give you value; your inverter is "too big" for such a low-use load.

I have not ruled out microinverters, but it seemed more complicated to add a battery. I'm hesitant to go Enphase as a relative has had a 20% fallout rate over the years with the IQ7s. I guess I do not understand the complete system on how it would interact of what you are saying. Microinverter, inverter, and battery? Are you stating to just have micro to lower bill, and use the grid to charge battery? I was picking one larger inverter as I appeared cheaper than multiple smaller inverters.

 

[Shimmy]

I have not ruled out microinverters, but it seemed more complicated to add a battery. I'm hesitant to go Enphase as a relative has had a 20% fallout rate over the years with the IQ7s. I guess I do not understand the complete system on how it would interact of what you are saying. Microinverter, inverter, and battery? Are you stating to just have micro to lower bill, and use the grid to charge battery? I was picking one larger inverter as I appeared cheaper than multiple smaller inverters.

Any grid-forming hybrid inverter can provide a grid reference for microinverters, or you can just have a small dumb inverter/charger to support your more critical loads. It's kind of a half-solution that makes sense in the short-term; the problem is that if you decide you want a 48VDC system the savings on the "cheap" system are limited.

You might take a look at @upnorthandpersonal 's system description: https://www.purplealienplanet.com/node/72 I might not recommend DIY battery personally, but we all have different constraints.

 

[skorch]

Any grid-forming hybrid inverter can provide a grid reference for microinverters, or you can just have a small dumb inverter/charger to support your more critical loads. It's kind of a half-solution that makes sense in the short-term; the problem is that if you decide you want a 48VDC system the savings on the "cheap" system are limited.

You might take a look at @upnorthandpersonal 's system description: https://www.purplealienplanet.com/node/72 I might not recommend DIY battery personally, but we all have different constraints.

Thanks Shimmy, I have contemplated making my batteries simply due to cost like that referenced website. I just wanted to settle on inverter setup first. I don’t know that I have done that yet, but Hedges appears to like Sunny Boy. Just hoping he responds to a couple questions regarding inverter size. I also need to find a way to reduce well pump load.

 

[Shimmy]

Hedges appears to like Sunny Boy.

I prefer the low frequency inverters like Schneider XW-Pro 6848 just for another data point, especially for motors. It's been around for over 10 years (IIRC). The unit can handle up to 10kW for motor starting, and 6.8kW for continuous. If you can make a water storage tank work though it will make for a more robust system.

 

[Hedges]

I believe you are correct, I think with a 200A service I have to look again but can go up to 10kW.

If you have a meter with 200A disconnect and separate panel, avoids NEC's 120% rule for panels. Utility sets the limit.

200A panel with 200A main breaker, 200A x 120% - 200A = 40A PV breaker allowed, 32A continuous, 7680W (or 7.7kW)
225A panel with 200A main breaker, 225A x 120% - 200A = 70A PV breaker allowed, 56A continuous, 13440W

Could you please tell me why I have seen people prefer to go with multiple small inverters instead of one larger inverter? It seems more cost efficient to purchase and install a larger one.

Redundancy is good if system operates through failures, or you can reconfigure around what's still working.
Enphase has been successful convincing people to get microinverters.
String inverters like Sunny Boy are reliable, and a larger system with multiple would provide redundancy.
Having 2 Sunny Islands, could be good to have a 120/240V transformer that could be connected to make system work if one failed.

I prefer the low frequency inverters like Schneider XW-Pro 6848 just for another data point, especially for motors. It's been around for over 10 years (IIRC). The unit can handle up to 10kW for motor starting, and 6.8kW for continuous. If you can make a water storage tank work though it will make for a more robust system.

Sunny Island is similarly low-frequency (and can be stacked series/parallel for more power and split phase.

Sunny Boy is high voltage string inverter, so high frequency. Older models had transformers, newer do not. It isn't the source of starting surge, which Sunny Island provides until Sunny Boy ramps up power in a few seconds.

Sunny Boy Storage appears to be a Sunny Boy programmed for battery connection. 6kW, 9kW surge, not stackable (yet.) Requires separate auto-transformer and transfer switch.

In Europe there is a new hybrid model, looks like a Sunny Boy with some MPPT inputs for PV, some for battery, produces 3-phase.

I think the battery is not going to be justifiable on economic terms, so you need to shift that into the "project requirements" bucket. Be aware that an inverter that can do 7.5kW likely requires a minimum of 7.5kWh of battery, and likely closer to 10kWh. That is going to be at least $3k of your budget. Adding $4-5k for the inverter and charge controller you are left with $7k for balance of system.

SMA recommends 100 Ah at 48V per Sunny Island, or 100 Ah per 1kW of Sunny Boy.
It will work with less, e.g. 100 Ah for a multiple Sunny Island multi kW Sunny Boy system, but might have problems of many kW of load suddenly switched off.
I use 400 Ah for 4x Sunny Island and 10kW of Sunny Boy. The batteries are only there for the starting surge, and to run minimal loads at night. SunXtender AGM, can discharge at 1C for 20kW (and should support brief surge higher)

If you use lithium, need to pay attention to max current of BMS.