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I have a small roof that barely fits 12 panels, a row of 8 in portrait on top, and a row of 4 in landscape below. The 4 will get some shadow on the bottom for about 4 months of the year, so they would need to go on a separate string ideally, however 4 panels is too few for most inverters wit double MPTT's.

For this reason I have been looking at these micro inverters:

Another feature that I would like is readily available on many cheap (hybrid) string inverters. An EPS (Emergency Power Supply) back-up feature that is intended to power the refrigerator and some lights (no heavy load) when the grid goes down. For example the Growatt SPH series.

With micro-inverters I could compensate by running the refrigerator and freezer on a regular UPS (usually used for computers, giving them time to shut down), for example from APC. However this will only last a short time since the micro-inverters will not feed the UPS.

Enphase has an extremely expensive solution, that ties you in to using their products, and another thing that rules them out is that providing internet to their system is almost compulsory in order to get it up and running or to clear a fault detection. I'm not planning to give them access to my NAS or allow packet-sniffing on my internal network any time soon.

AP-systems just say they do not support any back-up system at this time in their FAQ.

APsystems’ microinverters are designed to be grid-tied only. APsystems does not support using microinverters in conjunction with a generator on a closed-loop system. The APsystems warranty policy clearly states that microinverters used with a generator are not covered.

NEP does not mention back-up at all.

If I shut off the grid and all breakers except for one, (in fact shut off the whole house except a few outlets on the attic). Could I use a Power Station (portable battery inverter), for example like this one, to trick the micro inverters into delivering power again? Yes I know I would have to come up with something safer than a suicide cable, and that it would void APsystems warranty, but when the dikes are broken and I'm confined to the top floor I would not mind.

The question is, would it work? Which one would work best (AP / NEP)? Has anyone tested it?

If not I'll probably look for a small hybrid string inverter for the top 8 panels and Microinverters for the bottom 4...

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  • most of your post has nothing to do with the title question
    – jsotola
    May 12, 2023 at 0:48
  • Have you considered using a more "open architecture" system design? You can use micros (even the Enphase ones) in a backup capable system without the fancy boxes Enphase uses, you just need a different solution for your grid forming inverter May 12, 2023 at 2:00

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First, if first-floor flooding is something you need to think about, absolutely forget about using AC power unless your entire service is set up for that to work. Your service drop must be overhead not underground. All your service equipment including the meter must be on the top floor. Every circuit that goes downstairs must have GFCI at the breaker, not the receptacle so it trips out upstairs and does not energize wires that could get wet. Exactly that is done in a lot of housing in Houston, TX. Back to your question,

The redeeming quality of microinverter panels is they are a silver-bullet solution for Rapid Shutdown without weird and complex hardware - pull the meter and the panels are rendered safe. Can't beat it. The trade-off is they are an absolute nightmare for grid-down usability of those panels. By using microinverters, you are making life as hard as you could possibly make it for yourself.

Now there actually is a silver-bullet solution for grid-down use of microinverter panels. It's a bit of a "moonshot" technologically, but they figured it out. It's called a "Grid Forming Inverter" and it is able to trick the microinverter solar panels into powering up.

So your question is: is a "Grid Forming Inverter" just a cheaply made (but weirdly overpriced for what it is) inverter off Banggood, but with a fancy designer label and price tag?

No. It is not.

There's a lot more going on there, and if you want the capability you'll have to pay for it.

Now, if you want to avoid both microinverters and Rapid Shutdown problems, you can wire a DC array and design it to have an open-circuit voltage of less than 30 volts. Rapid shutdown and many other rules do not apply to voltages that low.

Now it's a simple matter: feed that into any common MPPT controller appropriate for that voltage, which charges really any battery, which feeds any common inverter appropriate for that voltage. Easy stuff, nothing weird and you have local power.

To have that array dump to the grid, add any common, run of the mill "grid tie inverter" and connect that either to the MPPT's "Dump" terminals, or simply to a knife switch so the array serves the MPPT or the grid-tie.

Now such a low voltage array requires positively enormous wires, and that's where you reach for aluminum. We are in a very low voltage system here, so the usual rules about solar array wiring don't apply.

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  • Thanks, I'm aware of your top paragraph and have a small breaker box with GFCI, breakers and a large manual "work switch" that will disconnect everything (except grounding) from the lower floors. Could you elaborate on "a lot more going on". As for now I picture it as an Arduino generating a sine wave with PWM and feeding it to a thyristor or something like that. This is not my field so please bear with me. I did a quick search on "grid-forming" but cannot find much more than "stable frequency" which does not really sound like a challenge... Would you have a link to a introductory resource? May 12, 2023 at 22:21
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Stop thinking like a box-pusher

The problem you're having is that you're thinking like a box-pusher who expects everything to be system-integrated for you. Instead, with backup, you'll want to at least look at doing your own integration work, with a fully hardwired grid-forming/multimode inverter or AC battery system that can then operate AC-coupled to any string or microinverter system that supports frequency-watt or volt-watt control. (Examples of the former include the Victron MultiPlus, Schneider Conext, and Outback Radian product lines -- these are all designed to work with bog-standard 24/48V deep cycle battery banks, but many can be used with Li-phosphate monoblocs or 48V Li-Ion modules as well, and can also DC couple to dedicated MPPT charge controllers. The latter category consists of the Powerwall and similar products.)

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    I don't think "box-pusher" is a phrase that is universally understood.
    – Tiger Guy
    May 12, 2023 at 14:53
  • I actually prefer separate components. It's the shading that causes me to consider micro-inverters, another choice would be solar-edge, but they give me the same gut-feeling as enphase for the same reasons... Any way, thanks for the examples and keywords. I will look these up and do more research. I feel like my "dead-end" is being opened up a little now I have some alternative products to look up. I already looked up "box-pusher", seems to be a snow shovel... never mind. May 12, 2023 at 22:21
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    "box-pusher" is a pejorative for a salesperson who only knows how to sell one company's pre-integrated "black box" solution, instead of a system integrator who can make multiple products work together May 13, 2023 at 2:32

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