Use of OFF-Grid-Only AC Coupling with MicroInverters? Good or Bad Idea?

keepdoingkeepdoing Registered Users Posts: 4
I'm a bit frustrated.  I want a totally off-grid system.  I want simple and reliable.  I want an auto-backup generator.  But I am not an electrician, nor an exceptional DIY guy that can do this stuff myself.  So I have now run through almost every Solar Installer in the North Georgia Mountain area - and still can't manage to find anyone willing to put together what I (ME) the Customer wants.

Everybody wants to sell me Grid-Tied.  The latest proposal is the subject of this post.  The proposed system is for an "off-grid" system that involves ..... an approximately 4500W Array with GT500 Microinverters AC Coupled to a Magnum 4448PAE, a 14kw LifePo4 Battery System, autobacked up by a 3kw or 7kw Honda Generator that is retrofitted to be powered by propane, fed by a 1000+ Gallon Propane reservoir.  Any excess power capacity would be fed to a Dump (an unused water heater most likely).

I know that tech has come a long way.  I know that these sort of AC Coupled "off-grid" systems have come about largely because "Grid-Tied" system sellers have increasingly met resistance from customers that want to use the generated Solar for Emergency Backup power, and don't understand that when the grid goes down on a traditional grid tied system - so does your solar power.  So this has emerged as an increasingly promoted solution..... for TEMPORARY emergency backup into an offgrid situation in the event of grid power outage.

I have done a LOT of reading and research on this, and this is generally what I have found to be concerns, concerns that might also be greatly exacerbated if I were to use this configuration for a 100% Fulltime PERMANENT Off-Grid system.....
  1. It's overly complex and therefore increases the odds of system failure (including battery damage to overcharge).
  2. Not only is it complex, it can involve very complicated variable settings, some which may require ongoing tweeking depending on how I am using the system at any given time. (I will have greatly variable use cases - sometimes as a weekender, sometimes full time, and quite possibly as a long term emergency situation if the grid goes down for a lengthy period due to bad actors)
  3. The added cost is not a justifiable ROI (I also have a full sun roof with no shade issues ever except stray clouds)

I think I know the answer to my own question here, but I am really getting worn down in this process, and want to throw this out to a hopefully impartial group of users for feedback.  What are everyone's thoughts on combining AC Coupled Microinverters into a 100% OffGrid system???

PS... also, any separate issues with using it with the LifePo4 batteries?

FYI: my primary goal is to have a solid and reliable system in event of SHTF scenarios.  There won't be any technicians coming to the rescue etc.

Thanks in advance.

- david h


  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    Welcome to the forum David... Answering your high level questions first.

    Using LiFePO4 batteries--Becoming cost effective (higher up front costs, seem to be longer cycle life). If you have high surge loads and low ongoing loads (i.e, microwave and water pumping, not much lighting/radio/TV/computer)--Lithium batteries are light weight and you can get away with a smaller AH battery bank. If you have a more balanced system (lights, more extensive water pumping, washing, TV/Computer/etc.--You need more AH battery bank just to store all of the energy you use--And bank will be large enough to support surge loading). Whole question of BMS (Battery Management System).

    BMS systems... Lithium batteries tend to "balance" better than Lead Acid batteries, but are more easily damaged with over/under charging than flooded cell lead acid batteries. Therefore, for Li Ion batteries, BMS systems are more or less required for safety (LiFePO4 batteries are safer--And folks do run without BMS, but must check balance with a volt meter weekly/monthly/when cycling to ensure balance).

    BMS systems... If you choose to get a bank with BMS, there are a few off grid systems that are starting to integrate BMS with their inverter/ charging systems. Still a bit cutting edge (not many vendors, Beta Testing, etc.). Can be quite the discussion if you want to get into details.

    A comment about water pumping... There are multi-power input pumps (AC, DC battery, Solar)--They are really nice (Grundfos, others) that can be setup to be "solar panel only" (pump to cistern), then use small/simple pump from cistern to home/irrigation. "Solar Friendly" water pumps are not cheap ($2-3k+?), but keeps your solar+battery+inverter system much smaller.

    I am not a fan of using Micro-Inverters with Off Grid (only) systems. I would suggest an integrated MPPT Solar charger + OG Inverter system--I.e., Schneider, Outback) (plus BMS integration, if Li Ion).

    Looking for low maintenance... AGM, LiFePO4 batteries are nice. If long life (15-20+ years same batteries), Forklift Traction batteries are hard to beat. However, you have to monitor electrolyte levels/have many gallons of de-ionized water on hand (Forklift batteries tend to use more water, and slightly less efficient--need a bit larger solar array to "cover" extra losses).

    Other batteries like Nickle Iron (Alkaline) batteries are "rugged". However, you do have to flush old electrolyte and replace with power+water every hand full of years.

    Typically, Lead Acid Batteries 5-8 years life (hot batteries, shorter life). AGM batteries maybe 1-2 year less life typical.

    Electronics (charge controllers, inverters, etc.)--Perhaps repairs at 5+ years. Start thinking about replacement at 10+ years (old electronics are generally not repairable or worth repairing).

    Auto backup Genset--A real mixed bag. If you are there--I would suggest manual start genset--There are so many wild and wonderful ways an autostart system can fail. But can be done if you wish.

    I like to start with your loads (Watts, Watt*Hours) to define your battery bank (typically 2 days of storage, 50% max discharge for longer battery life). Then define Solar array + Genset for charging to "keep the battery bank" happy.

    Anyway... A start. Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,198 ✭✭✭✭✭
    Agree with Bill that micro inverting is a bad idea in small applications offgrid.  Are you really going to be offgrid? No utility? I did not see this answer in your post.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • EstragonEstragon Registered Users Posts: 3,127 ✭✭✭✭✭
    My reading is OP wants a system that can be off-grid, either because he chooses to be or because the grid goes away.

    I considered but ruled out microinverters for my cabin. I'm a fan of keeping things as simple as possible (but no simpler... no off-grid system is as easy as flipping a switch on grid power).
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • keepdoingkeepdoing Registered Users Posts: 4
    Thanks to everyone for the responses.  I think I have ruled out MicroInverters.  I have also just run through yet another Solar Salesman since I posted this this morning.  This fellow drove almost 2 hours from Chattanooga, even though I clearly said I wanted NO Grid Tie, and a dedicated OffGrid only system.  He showed up and tried to sell me a Solar Edge System based on "battery backup" as an acceptable temporary solution to my stated scenario of "Complete Grid Failure".

    Estragon is correct.... this is primarily for the event that we suffer catastrophic grid failure.  But I am not a total idiot, and am planning the system so that I can use the power generated in the lucky event that we don't destroy ourselves.  Here is my situation and goal:

    I have a 2 level home/farm on a river in the mountains.  I just totally gutted the walkout basement and am remodeling it with a full farm kitchen, large bunkhouse sleeping room, one large bath, some additional living/storage rooms.  It's completely underground on 3.5 very energy efficient from heating/cooling.  The Upstairs is our normal living space.  It will continue to be on-grid.  The basement will be completely wired to the grid via local code also - BUT, I am running a complete secondary wiring system in the basement (including the ceiling fans and lighting) that will ONLY be tied to my offgrid solar system.  I will plug my Freezer, Fridge, Dehumidifier, Ceiling Fans & Lights to the Grid Sockets and those are the only dedicated loads I will have while the grid is still operational.  We don't live here full time anymore - but come over a couple of times a week to deal with farm issues, and occassionally spend a few days.  IF the grid goes down catastrophically - we'll move back full time and use the basement as our main living space / comfort zone.  Since we won't have direct solar wiring upstairs, we'll utilize portable battery packs for each bedroom for basics like lighting.  These are retrofitted mobile tool boxes with 12V 100AH LifePo4 batteries plus 80Amp Chargers and 1600W Inverters.  They will be charged every couple of days from wall sockets powered by a manual transfer switch to the generator, which we'll start manually.  But I want the second larger generator to autostart at certain low load levels and charge the main basement offgrid battery bank that runs those critical systems.  We'll have a minisplit on 110V cooling the basement - powered by the offgrid system.  I've designed the central HVAC so that the central circulation fan also runs off the Solar Circuitry (forgot to mention that as a dedicated load earlier) - and then a Master Return Vent int he basement I can unseal during outage and that will pull and circulate cool air housewide.  Won't be as cool as we get now, but it will be an improvement over what people with ZERO cooling/power have.  Hot Water, Cooking, and backup heating is Propane.  Shitload Lots of propane.  I'll also have a woodburning stove to dump heat in through a single wall vent.

    MY PROBLEM is that I come from the tech world and know too much about what is going on behind the scenes, especially in security and Artificial Intelligence fronts.  We ARE going to have a catastrophic failure at some point in the next few years.  I'd bet everything on it.  I was one of the early investors in Bitcoin, so I am lucky enough to suddenly have some excess cash.  Figure that this is a good way to spend it.  I hope I am wrong.

    To respond to Bill, who I really appreciate your comments as they were helpful.... I am going with Simpliphi Batteries I think (which has BMS) and something like the following configuration from wholesale solar.

    I actually talked to one of their guys (wholesale solar) for a long time today after my disaster with the Solaredge guy.  He has sort of convinced me I can do this myself with the help of an electrician (have a friend who can help but has never actually done solar).

    I think I am going to do this!  Finally after years of wishing for it.  So if anyone has concerns with this design, suggestions for improvement I welcome it.  I'm trying to make a decision by end of week.  One thing I am interested in is the earlier comment about going with manual start vs autostart on generator.  I welcome specific problem areas I could run into.

    I've spend loads of time lurking quietly on this forum over the years.  If I manage to pull this off succesfully it will be in no small part because of all the wisdom I have quietly absorbed here over the past year.  Thanks to everyone.

    - david

  • EstragonEstragon Registered Users Posts: 3,127 ✭✭✭✭✭
    Why do you need you need a decision by end of week? Anything we should know about :smile:

    The problem, as I see it, is the problem isn't well defined or quantified. If nobody wants to sell you a SHTF off-grid system locally, we (or NAWS) can certainly help you design one, but we need to know the parameters. What do you want to run in a SHTF scenario?
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,198 ✭✭✭✭✭
    The store here sells simplyphi also. I do not like that they do not output Soc. One needs an external battery monitor and shunt and programming to make it work.  I do not really like this approach as I would want internal BMS data. I think it is better and easier.
    As Estragon said above! What in the heck is the rush? Do you have a crystal ball or something?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • mcgivormcgivor Solar Expert Posts: 2,501 ✭✭✭✭✭
    edited May 2018 #8
    Quiet the undertaking for a first time endeavor, having an integrated system would probably be better than a series of mismatched components, speaking as an electrican I would say most electricians have very little direct experience with solar being somewhat of a niche market discipline. Perhaps contacting someone in the engineering department of SMA, Outback or Schneider, to mention a few, would be of more help than a sales person. Included pdf describes AC coupling it's advantages, disadvantages and so forth, something I was looking into for the future. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • mike95490mike95490 Solar Expert Posts: 8,067 ✭✭✭✭✭
    So, if you currently have grid power, your local zoning rules and power agreement clauses, may prevent you from disconnecting from the grid.    The term is called Grid Abandonment.   Some cities will red tag your house (unfit for human habitation) some may fine you $20K  for Abandonment fees.  Ir you may just disconnect.

    Regardless, the style inverter you want is a Hybrid Inverter, designed from the ground up, to work with batteries AND the Grid, whichever you set as priority.   Some can be configured for "Net Zero" so they do not feed back power into the grid.    If you do have grid, and install an inverter, you generally have to notify the electric company that you have done so, and you may need a inspection.

    When you add batteries, you are setting an expire life on your system, the batteries eventually fail, seldom lasting more than 10 years regardless of being used, or idle.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • Dave AngeliniDave Angelini Solar Expert Posts: 4,198 ✭✭✭✭✭
    I have always thought what Mike said makes sense! As Bill the moderator does at his home. I would modify his approach with using a propane generator and getting a large tank if I was rural and had outages.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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