diy solar

diy solar

Confused About AC vs. DC Coupling

Prior to this I was thinking you could just split the grid feed into 4 (for a 4 XW install) using a small box and 60 amp breakers and wires to each XW. Makes me wonder if that would work and each XW would switch their own 60amp relays when grid power is lost.

So are they trying to sell the BCS or is it required for grid tie with more than 60 amp service?

My understanding is Schneider can't count on all relays closing at same instant, so could overload the ones that do close.

SMA now has external 200A relay for European Sunny Island (and they had that in a box with auto-transformer for single 120V US Sunny Island), but otherwise they do parallel multiple units (56A max each.)

I have 4x SI, wired 2p2s. I figure I can't count on perfect division of current, 56A x 2 = 112A, so I set 90A limit.
With 40' of 6 awg wire and two QO270 breakers, I expected resistance to keep current balanced. It was imbalanced 3:1. That turned out to be the breakers which differed. Then I switched to Multi-9 63A DIN rail breakers and it matched within 10%. Now using 60A Midnight/CBI magnetic-hydraulic breakers (no resistance element for thermal) and it matches even closer.
 
Watching this video at 19 minutes they are talking about the BCS which is connected to the load and is basically a 200amp auto transfer switch "to keep from overloading the 60 amp relays" ... and their diagram is of directly connected to the grid.


When they are pointing out it not being an "on grid" they are no saying it can't be grid tied, but they seem to be saying they aren't meant to be directly connected to the grid before the main panel. All the diagrams show it being fed into breakers unless the BCS is used. But even then it isn't like the Sol-Ark and EG4 which connect to the grid feed on one connection and the load feed on another... so it requires the BCS to combine into a 200amp relay.

Prior to this I was thinking you could just split the grid feed into 4 (for a 4 XW install) using a small box and 60 amp breakers and wires to each XW. Makes me wonder if that would work and each XW would switch their own 60amp relays when grid power is lost.

So are they trying to sell the BCS or is it required for grid tie with more than 60 amp service?
In the video, there is a lot of details of what the BCS does that is not described well.

This may better be described as a GT system that allows multiple XW inverters to be AC coupled into main breaker panel similar to the way multiple PV grid tie inverters are connected to main panel. The XW's are acting more like PV GT inverters with battery power sourcing instead of PV power sourcing.

It uses the XW generator AC input port to sync inverter to grid freq/phase/voltage before taking inverter out of tri-state standby into active mode, just like what is done with a PV GT inverter. The tri-state standby of XW inverter allows ACout of inverter to be wired to main panel with grid source present while gen input sensing is setting up inverter's controller to proper freq/phase/voltage grid match required to bring up inverter from tri-state standby to active mode. It could also be using AC1 to connect inverter output to main panel but that is not clear from video. Using ACout from standby mode saves the pass-through relay power.

I do not see where it solves the sudden excess PV power situation when no grid is present, and a large AC load is switched off.

It looks like an expensive band-aid.
 
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Watching this video at 19 minutes they are talking about the BCS which is connected to the load and is basically a 200amp auto transfer switch "to keep from overloading the 60 amp relays" ... and their diagram is of directly connected to the grid.


When they are pointing out it not being an "on grid" they are no saying it can't be grid tied, but they seem to be saying they aren't meant to be directly connected to the grid before the main panel. All the diagrams show it being fed into breakers unless the BCS is used. But even then it isn't like the Sol-Ark and EG4 which connect to the grid feed on one connection and the load feed on another... so it requires the BCS to combine into a 200amp relay.

Prior to this I was thinking you could just split the grid feed into 4 (for a 4 XW install) using a small box and 60 amp breakers and wires to each XW. Makes me wonder if that would work and each XW would switch their own 60amp relays when grid power is lost.

So are they trying to sell the BCS or is it required for grid tie with more than 60 amp service?
The BCS is optional. I have set up a triple XW system with no BCS. (This was before the BCS was even available.) That system is still purring along with no issues whatsoever. That one is off-grid with a genset auto starting when batteries get low. Now, we did create an external transfer using contactors with a timed voltage sensing relay that would transfer loads in a way that avoids putting stress on the relays in the XWs.

Anytime you stack inverters, you run a risk of burning up transfer relays. Schneider specifically recommends not doing more passthrough amps than a single XW's passthrough rating (60A), even when stacking units.

What the BCS does is controls the connection to grid via a large contactor. When the inverters sense grid power (sensing is done at gen terminals) they synchronize up to the grid source, then give the signal for the BCS to pull the contactor. That connects grid into the loads panel, and then the XWs shift from inverting to charging via the wires from load terminals to panel. This eliminates any internal transfer, and all transfer make/break load gets managed by the contactor in the BCS.

This setup can be done using a contactor and your own controlling, but the BCS makes it way easier! We did a 3 phase setup this way once. The only thing that we did not set up was the WattNode meter (this is integrated in the BCS). Without the WattNode meter, you can't limit the amount of draw coming from grid. With the WattNode meter you could probably even do a "limited to home"/ zero export type of operation, since the WattNode meter tells the inverters (via Insight Home or Insight Facility) how much current is coming from, or going to, the grid.

Here is a screenshot of the drawing on the datasheet that shows the general gist of the power connections.
1710870541879.png

Here is a page from the XW Pro Multi-unit Design Guide:

1710870694060.png

And here are drawings showing how it is done with an external contactor, instead of the BCS.

1710870800157.png1710870855217.png

You can install up to 4 XW Pros on 120/240V split phase, with or without BCS or external contactor. The inverters will work fine and they can transfer internally all together, but you will very likely end up with burnt relays eventually.

Here is a page showing the issue with parallel units and passthrough current:

1710871310031.png
 
The BCS is optional. I have set up a triple XW system with no BCS. (This was before the BCS was even available.) That system is still purring along with no issues whatsoever. That one is off-grid with a genset auto starting when batteries get low. Now, we did create an external transfer using contactors with a timed voltage sensing relay that would transfer loads in a way that avoids putting stress on the relays in the XWs.

Anytime you stack inverters, you run a risk of burning up transfer relays. Schneider specifically recommends not doing more passthrough amps than a single XW's passthrough rating (60A), even when stacking units.

What the BCS does is controls the connection to grid via a large contactor. When the inverters sense grid power (sensing is done at gen terminals) they synchronize up to the grid source, then give the signal for the BCS to pull the contactor. That connects grid into the loads panel, and then the XWs shift from inverting to charging via the wires from load terminals to panel. This eliminates any internal transfer, and all transfer make/break load gets managed by the contactor in the BCS.

This setup can be done using a contactor and your own controlling, but the BCS makes it way easier! We did a 3 phase setup this way once. The only thing that we did not set up was the WattNode meter (this is integrated in the BCS). Without the WattNode meter, you can't limit the amount of draw coming from grid. With the WattNode meter you could probably even do a "limited to home"/ zero export type of operation, since the WattNode meter tells the inverters (via Insight Home or Insight Facility) how much current is coming from, or going to, the grid.

Here is a screenshot of the drawing on the datasheet that shows the general gist of the power connections.
View attachment 203031

Here is a page from the XW Pro Multi-unit Design Guide:

View attachment 203032

And here are drawings showing how it is done with an external contactor, instead of the BCS.

View attachment 203033View attachment 203034

You can install up to 4 XW Pros on 120/240V split phase, with or without BCS or external contactor. The inverters will work fine and they can transfer internally all together, but you will very likely end up with burnt relays eventually.

Here is a page showing the issue with parallel units and passthrough current:

View attachment 203036
Thank you very much, that unconfuses me and answers all my questions at once
 
In regards to the original question.

  • AC coupled is where the solar panels feed a standard Grid-Tie (GT) inverter, then the GT inverter is connected to the load side of the battery based inverter. While on-grid the AC power from PV is fed backwards through the battery based inverter's transfer relay and out to the grid. (You need to be set up for net metering for this.)
  • DC coupled is when your PV feeds charge controllers, that charge the battery bank that your battery based inverter is connected to. If you used a Schneider XW (or also some of the Outback inverters, and maybe other brands as well) you can set the battery based inverter to sell the excess power back to the grid. (Again a scenario that you need a net metering agreement.) When DC coupled, you can also choose to not sell to the grid, but operate similar to off-grid and cycle your batteries to provide your overnight power, falling back to grid when the PV and batteries run behind.
The new All-In-One inverters are designed to have the PV mppts built-into the battery based inverter. This gives you a much more clean install, with less components. It does come with a cost, however. Many of the AIOs do not have the surge that the battery based "low frequency" inverters do. Therefore surge loads can trigger an overload. There are a few units that do well, but there are many that do not! Sol-Ark 15Ks do very well! We have installed quite a number of them in whole house backup setups with great success in terms of surge capability! I believe the Eg4 18kPv does pretty well too, though I have not set one up myself. Both of those units have 200A transfer relays, so less likely to fail compared to 60A on say an XW Pro.

Most AIO inverters are closer to AC coupled topology, though not truly AC coupled. The PV goes from the mppts internally straight to a high voltage DC bus, then from there it either gets turned into AC and fed to loads and/or grid, or it gets bumped down to the battery voltage and charges the battery bank. Often you can AC couple to an AIO inverter, but normally you will have as good or better efficiency using the AIO's mppts, plus you have one less inverter (the GT inverter) to potentially fail!

A few points to consider:

  • When off-grid your efficiency is normally quite a bit better going DC coupled! I am off-grid and am AC coupled and my efficiency is not great! Only ~30% of my power gets used straight off the GT inverter's production, which means ~70% of my PV has efficiency losses of GT inverter > battery based inverters/charging losses> battery based inverters from battery to load. I would estimated that efficiency to be around 70-75%! DC coupled would only have charge controller to battery > inverter battery to loads losses. Though those losses would always be there. But I would guess DC coupled would probably net ~85-90% efficiency round trip.
  • AC coupled will be more efficient in a grid-tie with battery backup, where you are not cycling the batteries. Therefore your GT inverter can feed straight to AC "bus" (breaker panel and grid) under normal conditions. The only thing that can get interesting is to get the settings all fine-tuned to be able to keep the batteries fully charged via the battery based inverter (XW, etc.). That sounds like nothing major, but I have worked on systems where it was a struggle!
  • AIO inverters grid-tied.... For a grid-tie with battery backup option, AIO just makes sense. And even for a grid-tie with future battery option, AIO is the way to go! Using something like the Sol-Ark 15K or Eg4 18kPv will just plain keep everything very streamlined, while still having decent surge capabilities for when grid power is down!
  • AIO off-grid..... I'm honestly somewhat neutral on this one.... We install tons of AIOs (Sol-Ark primarily) off-grid! But there is a lack of off-grid functionality that can be very frustrating! Things like gen integration and auto starting not working very well or not being very practical have me second guessing these units for off-grid! And then there is the whole PV round trip efficiency thing as well. An AIO will have a similar round trip efficiency as my AC coupled system at home! Which makes the good old low frequency inverters with charge controller and proper generator compatibility and auto starting options look like the gems that they are in off-grid! They do make a clean install! And I do like them in many ways..... but theres those other things that are very infuriating! lol
Hopefully this helps you understand AC vs Dc coupled and the pros and cons!
 
In regards to the original question.

  • AC coupled is where the solar panels feed a standard Grid-Tie (GT) inverter, then the GT inverter is connected to the load side of the battery based inverter. While on-grid the AC power from PV is fed backwards through the battery based inverter's transfer relay and out to the grid. (You need to be set up for net metering for this.)
  • DC coupled is when your PV feeds charge controllers, that charge the battery bank that your battery based inverter is connected to. If you used a Schneider XW (or also some of the Outback inverters, and maybe other brands as well) you can set the battery based inverter to sell the excess power back to the grid. (Again a scenario that you need a net metering agreement.) When DC coupled, you can also choose to not sell to the grid, but operate similar to off-grid and cycle your batteries to provide your overnight power, falling back to grid when the PV and batteries run behind.
The new All-In-One inverters are designed to have the PV mppts built-into the battery based inverter. This gives you a much more clean install, with less components. It does come with a cost, however. Many of the AIOs do not have the surge that the battery based "low frequency" inverters do. Therefore surge loads can trigger an overload. There are a few units that do well, but there are many that do not! Sol-Ark 15Ks do very well! We have installed quite a number of them in whole house backup setups with great success in terms of surge capability! I believe the Eg4 18kPv does pretty well too, though I have not set one up myself. Both of those units have 200A transfer relays, so less likely to fail compared to 60A on say an XW Pro.

Most AIO inverters are closer to AC coupled topology, though not truly AC coupled. The PV goes from the mppts internally straight to a high voltage DC bus, then from there it either gets turned into AC and fed to loads and/or grid, or it gets bumped down to the battery voltage and charges the battery bank. Often you can AC couple to an AIO inverter, but normally you will have as good or better efficiency using the AIO's mppts, plus you have one less inverter (the GT inverter) to potentially fail!

A few points to consider:

  • When off-grid your efficiency is normally quite a bit better going DC coupled! I am off-grid and am AC coupled and my efficiency is not great! Only ~30% of my power gets used straight off the GT inverter's production, which means ~70% of my PV has efficiency losses of GT inverter > battery based inverters/charging losses> battery based inverters from battery to load. I would estimated that efficiency to be around 70-75%! DC coupled would only have charge controller to battery > inverter battery to loads losses. Though those losses would always be there. But I would guess DC coupled would probably net ~85-90% efficiency round trip.
  • AC coupled will be more efficient in a grid-tie with battery backup, where you are not cycling the batteries. Therefore your GT inverter can feed straight to AC "bus" (breaker panel and grid) under normal conditions. The only thing that can get interesting is to get the settings all fine-tuned to be able to keep the batteries fully charged via the battery based inverter (XW, etc.). That sounds like nothing major, but I have worked on systems where it was a struggle!
  • AIO inverters grid-tied.... For a grid-tie with battery backup option, AIO just makes sense. And even for a grid-tie with future battery option, AIO is the way to go! Using something like the Sol-Ark 15K or Eg4 18kPv will just plain keep everything very streamlined, while still having decent surge capabilities for when grid power is down!
  • AIO off-grid..... I'm honestly somewhat neutral on this one.... We install tons of AIOs (Sol-Ark primarily) off-grid! But there is a lack of off-grid functionality that can be very frustrating! Things like gen integration and auto starting not working very well or not being very practical have me second guessing these units for off-grid! And then there is the whole PV round trip efficiency thing as well. An AIO will have a similar round trip efficiency as my AC coupled system at home! Which makes the good old low frequency inverters with charge controller and proper generator compatibility and auto starting options look like the gems that they are in off-grid! They do make a clean install! And I do like them in many ways..... but theres those other things that are very infuriating! lol
Hopefully this helps you understand AC vs Dc coupled and the pros and cons!
Good info but original question was asked two years ago.

I’m off grid with all AC coupling as I can use that power during the daytime to operate more amperage than my system (4x Sunny Islands) would normally handle drawing straight from battery.
 
In regards to the original question.

  • AC coupled is where the solar panels feed a standard Grid-Tie (GT) inverter, then the GT inverter is connected to the load side of the battery based inverter. While on-grid the AC power from PV is fed backwards through the battery based inverter's transfer relay and out to the grid. (You need to be set up for net metering for this.)
  • DC coupled is when your PV feeds charge controllers, that charge the battery bank that your battery based inverter is connected to. If you used a Schneider XW (or also some of the Outback inverters, and maybe other brands as well) you can set the battery based inverter to sell the excess power back to the grid. (Again a scenario that you need a net metering agreement.) When DC coupled, you can also choose to not sell to the grid, but operate similar to off-grid and cycle your batteries to provide your overnight power, falling back to grid when the PV and batteries run behind.
The new All-In-One inverters are designed to have the PV mppts built-into the battery based inverter. This gives you a much more clean install, with less components. It does come with a cost, however. Many of the AIOs do not have the surge that the battery based "low frequency" inverters do. Therefore surge loads can trigger an overload. There are a few units that do well, but there are many that do not! Sol-Ark 15Ks do very well! We have installed quite a number of them in whole house backup setups with great success in terms of surge capability! I believe the Eg4 18kPv does pretty well too, though I have not set one up myself. Both of those units have 200A transfer relays, so less likely to fail compared to 60A on say an XW Pro.

Most AIO inverters are closer to AC coupled topology, though not truly AC coupled. The PV goes from the mppts internally straight to a high voltage DC bus, then from there it either gets turned into AC and fed to loads and/or grid, or it gets bumped down to the battery voltage and charges the battery bank. Often you can AC couple to an AIO inverter, but normally you will have as good or better efficiency using the AIO's mppts, plus you have one less inverter (the GT inverter) to potentially fail!

A few points to consider:

  • When off-grid your efficiency is normally quite a bit better going DC coupled! I am off-grid and am AC coupled and my efficiency is not great! Only ~30% of my power gets used straight off the GT inverter's production, which means ~70% of my PV has efficiency losses of GT inverter > battery based inverters/charging losses> battery based inverters from battery to load. I would estimated that efficiency to be around 70-75%! DC coupled would only have charge controller to battery > inverter battery to loads losses. Though those losses would always be there. But I would guess DC coupled would probably net ~85-90% efficiency round trip.
  • AC coupled will be more efficient in a grid-tie with battery backup, where you are not cycling the batteries. Therefore your GT inverter can feed straight to AC "bus" (breaker panel and grid) under normal conditions. The only thing that can get interesting is to get the settings all fine-tuned to be able to keep the batteries fully charged via the battery based inverter (XW, etc.). That sounds like nothing major, but I have worked on systems where it was a struggle!
  • AIO inverters grid-tied.... For a grid-tie with battery backup option, AIO just makes sense. And even for a grid-tie with future battery option, AIO is the way to go! Using something like the Sol-Ark 15K or Eg4 18kPv will just plain keep everything very streamlined, while still having decent surge capabilities for when grid power is down!
  • AIO off-grid..... I'm honestly somewhat neutral on this one.... We install tons of AIOs (Sol-Ark primarily) off-grid! But there is a lack of off-grid functionality that can be very frustrating! Things like gen integration and auto starting not working very well or not being very practical have me second guessing these units for off-grid! And then there is the whole PV round trip efficiency thing as well. An AIO will have a similar round trip efficiency as my AC coupled system at home! Which makes the good old low frequency inverters with charge controller and proper generator compatibility and auto starting options look like the gems that they are in off-grid! They do make a clean install! And I do like them in many ways..... but theres those other things that are very infuriating! lol
Hopefully this helps you understand AC vs Dc coupled and the pros and cons!

So, in my situation:
I want to net zero my system with just enough sell back to cover the connection fees
I need to stay grid connected to run a glass kiln occasionally 50amps for hours on and off
I intend a heat pump for heating, cooling, underfloor heating, pool heating.
Run heating for a hot tub (5 to 7 kw heater)
Tankless water heater preheat
Backup heat will be either boiler or forced air gas furnace.
might have a few split units - nots sure.

The house plans are still still being drawn up but I intend to have a power wall with all the equipment mounted. For the certificate of occupance I plan to get the contractor to use whatever I figure out for the install and once we move in I will DIY the battery, pannels, wiring, mppt, etc. This part will all have to be code compliant and with interconnect agreement.

What sort of unit(s) do you recommend?
 
Good info but original question was asked two years ago.
Oops, well maybe it'll still help someone else who is wanting to know the differences...
I’m off grid with all AC coupling as I can use that power during the daytime to operate more amperage than my system (4x Sunny Islands) would normally handle drawing straight from battery.
(y)
That's a big part of why I went AC coupled. I have 4x Victron 3,000/48 with a Fronius 7.6 AC coupled. I love the fact that when the sun is shining, my Victron barely even notice when the dryer kicks on! (That's 6kw of load!) But now what bugs me is when I look at my charts of how my PV power gets used and see the large percentage going round trip to the batteries! During the summer we have 2 window A/C units that use power as it's produced, and we also have an electric dryer. But I know our PV efficiency would be a lot better if we were DC coupled. And in the winter when my backup gen runs I think about it even more.


Here is a screenshot. And the "From Battery" portion would also include some additional gen power that is not reflected here, because this is just showing where my loads came from.
1710933824399.png


And here is a chart of my last 12 months. As you can see, I have some generator run time even in the summer (very minimal) and that efficiency difference would possibly make the difference of not needing any gen run time.

1710934588334.png
 
So, in my situation:
I want to net zero my system with just enough sell back to cover the connection fees
I need to stay grid connected to run a glass kiln occasionally 50amps for hours on and off
I intend a heat pump for heating, cooling, underfloor heating, pool heating.
Run heating for a hot tub (5 to 7 kw heater)
Tankless water heater preheat
Backup heat will be either boiler or forced air gas furnace.
might have a few split units - nots sure.

The house plans are still still being drawn up but I intend to have a power wall with all the equipment mounted. For the certificate of occupance I plan to get the contractor to use whatever I figure out for the install and once we move in I will DIY the battery, pannels, wiring, mppt, etc. This part will all have to be code compliant and with interconnect agreement.

What sort of unit(s) do you recommend?
I would lean towards Sol-Ark 15K(s). I am guessing for those loads you will be looking at 2 to 3 of the 15Ks. But you should get a total load calculation and work off of that for how many inverters. Just remember to look at the "out of the battery" numbers of whatever inverter you decide to use. (For the 15K that is 12kW out of the battery with no solar.) Most bigger AIO inverters have a nameplate rating that is with solar, but not achievable overnight with no solar contributing to load output.

If your preference would be to go with "low frequency" inverter, my personal preference is Schneider XWs. But I know some people really like the Sunny Islands, although I have no experience with them and I'm not sure how lithium friendly they are at this point. I don't work with them, though, so I may be wrong on that!

A few things to consider:
  • Get load calculations together to decide how much grunt power you need during a grid outage scenario. Work off that to decide how many inverters you need.
  • Decide if some of your loads could be in a non-critical loads panel that would just be "dead" during a grid outage. That would lower your peak power demand. (This could be things like hot tub heat, tankless water heater preheat, etc.)
  • If you do have some loads that are non-critical, you can always use external CTs to allow an AIO like the Sol-Ark 15K or Eg4 18kPv to use battery power overnight to offset those loads as well.
  • If your power company does net metering within the billing cycle (where they figure up the sell/buy difference at the end of every billing cycle and credit/bill accordingly) then use the power company as your batteries! That way you can get away with much less battery storage, as well as avoid cycling your batteries for longer battery life!
  • Your glass kiln may be something that you want to run non-critical to limit the total inverter power needed. Or you could also say that you won't run that when grid is down. But just remember that generally speaking, you will probably not even notice when grid goes off, if your inverter(s) are working like they should be, and the transfer is clean!
  • Be sure to have some kind of bypass option for that random time that your inverters have an issue, and you need power. Either have a manual double throw transfer switch, or if you can make do with 100-125A in a pinch, you can get away with an interlock between your main breaker and a bus breaker for backup. When the inverters throw a fault, the transfer relay in them will often also not pull!
  • When putting together a bigger system it is usually advisable to use bigger units for a few reasons. Bigger units will have "heavier" components. For instance, a Sol-Ark 15K has 200A transfer relay vs a 12K has only a 60A relay. So using 2x 15K gives you 400A of transfer vs 3x 12K is only 180A, or even 4x 12K is only 240A! If you go with a "low frequency" inverter like XW Pro, you will only have 60A option, but then I would do external transfer, either with the BCS or with your own components. Another benefit of using less, but bigger units, is less potential communication issue with less units stacked.
  • If you do end up going with an AIO inverter, I would try to use a battery that has closed loop comms, purely because the AIOs (in my experience) are terrible at tracking battery SOC internally! If you would go with XW Pros, the Schneider Battery Monitor works fine to track SOC if the settings are configured correctly. (You can do closed loop with the Insight Home or Facility, but they may be limited in the brands that they work with.)
Hopefully that helps you a little!
 
I have an 18 KPV as my main inverter, carrying the whole house. I have a Growatt 11 as my AC coupled inverter. Batteries on the 18. SO far, so good, and the Growatt seems to be better at harvesting energy than the 18, same size strings make more juice on the 11.

My goal is the same, basically net zero.
 
I have an 18 KPV as my main inverter, carrying the whole house. I have a Growatt 11 as my AC coupled inverter. Batteries on the 18. SO far, so good, and the Growatt seems to be better at harvesting energy than the 18, same size strings make more juice on the 11.

My goal is the same, basically net zero.
Curious how much PV on each inverter?
 
Signature has details, but all the PV is not yet hooked up to the 11.

Right now the 11 has 3600 of the Canadian Solar on it, and 2960 of the NE Solar PV on it.

When I'm done, the 18 will have 12,800 and the 11 will have 11,470.
 
I would lean towards Sol-Ark 15K(s). I am guessing for those loads you will be looking at 2 to 3 of the 15Ks. But you should get a total load calculation and work off of that for how many inverters. Just remember to look at the "out of the battery" numbers of whatever inverter you decide to use. (For the 15K that is 12kW out of the battery with no solar.) Most bigger AIO inverters have a nameplate rating that is with solar, but not achievable overnight with no solar contributing to load output.

If your preference would be to go with "low frequency" inverter, my personal preference is Schneider XWs. But I know some people really like the Sunny Islands, although I have no experience with them and I'm not sure how lithium friendly they are at this point. I don't work with them, though, so I may be wrong on that!

A few things to consider:
  • Get load calculations together to decide how much grunt power you need during a grid outage scenario. Work off that to decide how many inverters you need.
  • Decide if some of your loads could be in a non-critical loads panel that would just be "dead" during a grid outage. That would lower your peak power demand. (This could be things like hot tub heat, tankless water heater preheat, etc.)
  • If you do have some loads that are non-critical, you can always use external CTs to allow an AIO like the Sol-Ark 15K or Eg4 18kPv to use battery power overnight to offset those loads as well.
  • If your power company does net metering within the billing cycle (where they figure up the sell/buy difference at the end of every billing cycle and credit/bill accordingly) then use the power company as your batteries! That way you can get away with much less battery storage, as well as avoid cycling your batteries for longer battery life!
  • Your glass kiln may be something that you want to run non-critical to limit the total inverter power needed. Or you could also say that you won't run that when grid is down. But just remember that generally speaking, you will probably not even notice when grid goes off, if your inverter(s) are working like they should be, and the transfer is clean!
  • Be sure to have some kind of bypass option for that random time that your inverters have an issue, and you need power. Either have a manual double throw transfer switch, or if you can make do with 100-125A in a pinch, you can get away with an interlock between your main breaker and a bus breaker for backup. When the inverters throw a fault, the transfer relay in them will often also not pull!
  • When putting together a bigger system it is usually advisable to use bigger units for a few reasons. Bigger units will have "heavier" components. For instance, a Sol-Ark 15K has 200A transfer relay vs a 12K has only a 60A relay. So using 2x 15K gives you 400A of transfer vs 3x 12K is only 180A, or even 4x 12K is only 240A! If you go with a "low frequency" inverter like XW Pro, you will only have 60A option, but then I would do external transfer, either with the BCS or with your own components. Another benefit of using less, but bigger units, is less potential communication issue with less units stacked.
  • If you do end up going with an AIO inverter, I would try to use a battery that has closed loop comms, purely because the AIOs (in my experience) are terrible at tracking battery SOC internally! If you would go with XW Pros, the Schneider Battery Monitor works fine to track SOC if the settings are configured correctly. (You can do closed loop with the Insight Home or Facility, but they may be limited in the brands that they work with.)
Hopefully that helps you a little!

I appreciate the insights - Hard to do a energy audit on a house not built yet, but I can look at my current energy consumption as a guestimate - This is from the power company now - the major difference is lack of pool and lack of heatpump. Currently we live at 9144ft and it is cold for a lot of the year. We are moving to Cortez, CO which is about 6500ft. So instead of no air conditioning needed there will be several months where it is required. But, the hot tub won't have to work nearly so hard in winter to keep warm. The property we are looking at has power and water to the property line, but no gas or sewer. But at 3.5 acres there is more than enough room for a solar array.

The kiln, and hot tub would for sure be in a non-critical panel. So the goal is not to cover outages so much as to not have an electric bill monthly. They do a yearly true up of solar sold. Monthly it is a bank which can be used 1-to-1 later and the annual true up is at wholesale rates if I have a surplus.

I intend low frequency inverters plus a single grid tied ac coupled inverter. I really like the Sol-Ark 15k models but if I need to stack them then they could have the same issue as the LF models - single transfer relays carrying the entire load and burning contacts.

I like the XW Pro models but I was also thinking about a quad of Victron Multiplus II models.

Here we only have a 200amp service and once we move I expect it will be the same because most of the power would be from the solar array in the long run. But I will have to calculate it out after consulting the geothermal heat pump folks for sizing and how much the pump draws. I have read that they take a crazy amount of power to start and then are pretty efficient after that. Next on my list to research after solar panels.

I think I have a really good handle on the grid tie stuff now thanks to you.

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They do a yearly true up of solar sold. Monthly it is a bank which can be used 1-to-1 later and the annual true up is at wholesale rates if I have a surplus.
That sounds pretty good! Here in Ohio it is normally a monthly true up at wholesale rate!
I intend low frequency inverters plus a single grid tied ac coupled inverter. I really like the Sol-Ark 15k models but if I need to stack them then they could have the same issue as the LF models - single transfer relays carrying the entire load and burning contacts.
The biggest thing about the 15K, in regards to transfer relay, is they have 200A relays.
I like the XW Pro models but I was also thinking about a quad of Victron Multiplus II models.
I have a quad Multiplus II 3000. I absolutely love Victron! If you do go that route, I would look into the Quattros, as I believe those have 100A relays instead of 50A for the Multiplus II. Only thing though, is that if you need to have UL listing I think you are stuck with the Multiplus II. They do have th 5000w now though, which they didn't have when I built my system.

If you go with Fronius for your GT inverter, you will get a pretty good amount of integration between it and a Victron system!
Here we only have a 200amp service and once we more I expect it will be the same because most of the power would be from the solar array in the long run. But I will have to calculate it out after consulting the geothermal heat pump folks for sizing and how much the pump draws. I have read that they take a crazy amount of power to start and then are pretty efficient after that. Next on my list to research after solar panels.
I think a lot of the air source heat pumps anymore are inverter drive. I wonder if the geothermal ones are available in inverter drive as well. That gets rid of your startup surge!
I think I have a really good handle on the grid tie stuff now thanks to you.
(y)
 
That sounds pretty good! Here in Ohio it is normally a monthly true up at wholesale rate!

The biggest thing about the 15K, in regards to transfer relay, is they have 200A relays.

I have a quad Multiplus II 3000. I absolutely love Victron! If you do go that route, I would look into the Quattros, as I believe those have 100A relays instead of 50A for the Multiplus II. Only thing though, is that if you need to have UL listing I think you are stuck with the Multiplus II. They do have th 5000w now though, which they didn't have when I built my system.

If you go with Fronius for your GT inverter, you will get a pretty good amount of integration between it and a Victron system!

I think a lot of the air source heat pumps anymore are inverter drive. I wonder if the geothermal ones are available in inverter drive as well. That gets rid of your startup surge!

(y)

I will require everything to be UL listed and to meet NEC2020 currently. They usually run a year or so behind the current NEC so they may change to NEC2023 at any time. I have a few books on the topic so I can make sure I do it all right.

I have been looking hard at the SOK server rack batteries and the external 12 slot enclosure. That takes a standard telecom heating and cooling unit to keep the batteries good. I like it being separate but I am also thinking inside the detached garage and having that space wth a minisplit heating and cooling unit. We are planning a L shaped 3 car garage so 2 wide and 2 deep on one side. The back of the 2 deep would be a work room and electrical/utility area.
 
Ok, so I am kind of stumped. It seems I need an anti-islanding device. I need to have the full 200 amp grid service when the house is built. Once I move in I plan to do all the solar install and minimize the grid usage once that is up. Seems like any GTI inverter has way to small a capacity so or is intended to work with panels from the start. None that I look up have high pass-through current.

So what is a UL listed solution for that? I was hoping for an off the shelf device to just have installed between the grid entrance and the main load center so I could drop power later and do all the inverter and power work myself.

Can I just use an auto transfer switch with time delay for the job and leave the generator leg disconnected? Route the load leg down to an empty breaker panel then connect the main load center with all breakers in it off the bottom lugs. Then later snap
breakers Into the empty panel to connect a quad of LF DC coupled inverters into it? Room for more is needed?

Something like this "Eaton BR 200A 4-Space 8-Circuit Main Breaker Load Center" with 4 x 50amp 2-pole breakers for the feed through?

If I ever wanted to go completly off grid I could just turn the transfer switch.
 
Ok, maybe that is a bad idea, how would the inverters sync the frequency? Probably better to use a couple of boxes to configure bypass breakers and run with that until the inverter install. Then I can wire and connect everything and after the inspections it is just flip so switches.
 
Ok, so I am kind of stumped. It seems I need an anti-islanding device. I need to have the full 200 amp grid service when the house is built. Once I move in I plan to do all the solar install and minimize the grid usage once that is up. Seems like any GTI inverter has way to small a capacity so or is intended to work with panels from the start. None that I look up have high pass-through current.

So what is a UL listed solution for that? I was hoping for an off the shelf device to just have installed between the grid entrance and the main load center so I could drop power later and do all the inverter and power work myself.

Can I just use an auto transfer switch with time delay for the job and leave the generator leg disconnected? Route the load leg down to an empty breaker panel then connect the main load center with all breakers in it off the bottom lugs. Then later snap
breakers Into the empty panel to connect a quad of LF DC coupled inverters into it? Room for more is needed?

Something like this "Eaton BR 200A 4-Space 8-Circuit Main Breaker Load Center" with 4 x 50amp 2-pole breakers for the feed through?

If I ever wanted to go completly off grid I could just turn the transfer switch.
PV interconnection always gets interesting because of the NEC "rules" on interconnection. I will list a quick overview of how you are allowed to interconnect, as there are some specifics to keep in mind.

If you want to just pop a breaker into a main panel the rules are as follows:
  • Either- all bus breakers in the panel must total less than the busbar rating of the panel
  • Or- Pv breaker must be at opposite end of busbar from main and total of main breaker + PV breaker must be no more than busbar rating of panel x 120%. (For example- 200A rated panel x 1.2 = 240A total. So- 200A main + max 40A Pv breaker or 175A main + max 65A PV breaker.)
Your "Eaton BR 200A 4-Space 8-Circuit Main Breaker Load Center" would work as the main panel, however that would limit you to just those 4x 50A breakers (I assume you are talking about 1 pole for feeding Victron 120V inverters.). So then you would essentially be limited to 100A of total draw. And you couldn't legally use that panel installed as a sub panel off a separate main panel, because then your 200A breaker feeding this sub panel would be the interconnection point, and would not be legally allowed, if you look at the requirements that I listed above.

The other option is a line side tap. Some power companies don't like and/or allow a line side tap. A line side tap will be by far your best option for what you said you want to do. Basically what it is, is a separate service specifically for your solar. The wires feeding that service get connected to the wires between your meter and your main breaker in the service entrance panel. Then you go directly to an outdoor fused disconnect that gets grounded and bonded as a main service disconnect, and from there you could go to a panel that is specifically for you solar system.

If the power company does not want a line side tap, you could always just say you want a 400A service, and have 2 separate 200A outdoor fused disconnect for you service entrance. Then one would be your grid direct loads and the second one would be your PV disconnect that is lockable and meets NEC requirements. From the PV disconnect you would go directly to that Eaton panle that you mentioned, and because it would be full with those 4x 1 pole 50A breakers, it would be a legal interconnection. Those would feed through the inverters, then on the load side you would have critical loads panel, and once again, you will need to ensure that the panel will not be overloaded between the mains from those inverters, and the GT inverter breaker. So using a 200A main lug panel and again 4x 1 pole 50A breakers for the mains, you would have (200Ax 120%) 240A - 100A (those 4x 1 pole 50A) = 140A of headroom for GT inverter!

GT inverter breaker is sized as follows. Max amps rated x 1.25 == round up to nearest higher breaker size. For example, a Fronius 8.2 is rated 34.2A, so 34.2 x 120% = 41.05A. Round that up to 50A and that is your breaker size.

Keep in mind that when AC coupling to almost any battery based inverter, you need to keep a 1:1 ratio in rated wattage between GT vs battery inverter. So if you have 4x Victron 48/5000 multiplus II then your are limited to 20,000 watts total of GT inverter. So you could go up to, say, 2x Fronius Primo 10.0.

As far as an anti-islanding device goes, I believe that in the US when the Victrons are not actually the ones pushing the power out to grid (your GT inverter would be the one doing that.) you probably don't need a special anti-islanding device. Now do your research on this, as I haven't set up Victrons in an AC coupled Grid-tied scenario! At home I'm AC coupled, but off-grid.

I believe the Victrons disconnect quick enough to be legal, and during a power outage/grid disconnect scenario, the GT inverter would actually shut down for 5 minutes due to seeing the power fluctuate during disconnect.

I do know for sure though, that an inverter like the Schneider XW Pro would definitely not need any additional anti-islanding device.

Hopefully this helps, and doesn't make it confusing. It is always interesting figuring out the best (and NEC code compliant) way to interconnect! But I have found that generally speaking, if the PV system will need more than a 40A breaker, then a line side tap is almost always the only viable option after looking at all angles!
 
I was thinking to use 2 pole 50amp breakers in the eaton box. So 4 x 2 pole with inverters connected. And now after reading what you wrote I look agan and there are only 4 contacts sticking up.... so maybe I need to look for a feed through panel that is 200amps and takes 4 x 2 pole 30 amp breakers. So ideally I would feed 25 amps per leg per inverter


Something I read in another thread has me thinking as well.

A 200amp ATS, grid power wired to the generator leg and a panel with breakers to the solar system on the grid leg. The idea being it would be switched to grid unless the solar system was up and stable.

It means I could run from grid while the house is built and from solar anytime I have enough PV and battery power. So ideally I could right size the system and not ever flip to grid unless it has been cloudy multiple days. Using victrons I would have the surge power to support the load of the heat pump.

I have been thinking about ground mounts and I think I can make a 2 axis tracker for only about $200 to $300. 6 panel mount. I am going to study what is out there already and see if someone already has done it or if I have a million dollar idea. If nobody has one like what I am thinking I'll model it in 3d and print a few small scale ones to test out. Seems like tracking and getting more bang for the buck might be cheaper than more panels and equip. I also have an idea for doing that with a row of panels, but inbet someone else has already done it.

And we are looking at a house that is just a shell of new construction and the builder has 3200w panels, solark5k, and a battery at the moment. I think it is intended to be a totally off-grid house in the end, which would suit me fine.

Thinking out loud and much more reading to do... I'll do up a drawing to post and see what you think.


Well, scratch that idea, looks like smarter people than memhave already figured it all out. I guess I stay poor for now.
 
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