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I am installing a sungold power 12kW low frequency inverter that will use a battery array (and, eventually, also an array of solar panels) to provide power to a set of circuits in my home. This inverter also can be wired with an AC input to charge the batteries (either entirely, or supplementing the solar panels).
The inverter contains its own internal transformer and creates its own neutral. As such, the AC input is wired with two hots, no ground, no neutral. The output feeds to a service panel with the set of loads powered by the system. Per the manual and continuity testing, there is no internal neutral ground bond.

My question is as follows: should I consider this load center a main panel or a subpanel? In particular, shall I bond neutral to ground in the load center downstream of the inverter?

There is no return path to my home's main service panel ground, and I would expect I should add a neutral ground bond in the load center downstream of the inverter so that GFCI breakers will trip if there is a ground fault.
The manufacturer states that I should have a qualified electrician install the inverter and they will know what to do, but I would rather do it myself.

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    @DoxyLover yes, this is obvious, and stated in the question. If you have an answer re: whether I should bond it to ground in the load center, or whether the inspector might consider that a subpanel and I should instead find a solution that connects the ground back to the main service panel, that would be useful.
    – De Novo
    Jun 18 at 0:51
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    What make and model is the inverter in question?
    – ThreePhaseEel
    Jun 18 at 1:37
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    Sungold power 12kW. The UL listed model. I believe it's made by Voltronic.
    – De Novo
    Jun 19 at 1:30
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    Just for your awareness, GFCIs do not care about ground. They monitor hot(s) and neutral only, and open the circuit if there is an imbalance ("current out" not equal to "current returning"). They do not require a neutral-ground bond anywhere and would even work with a totally floating, ungrounded system. You should still have a neutral-ground bond in the correct place for safety, though.
    – nobody
    Jun 19 at 17:40
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    It sounds like you're building a "separately derived system" (electrical code terminology). While waiting for any answer(s) to be posted here, you might also do some research with that term to see what you may learn.
    – Greg Hill
    Jun 19 at 21:39

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This answer assumes NEC2020. There are some relevant differences in NEC 2023, like replacing the outside emergency disconnect for energy storage systems, with an emergency shutdown initiation system, and no longer requiring disconnects within 10 feet of the ESS to be lockable in the open position. And probably more too.

You have an Energy Storage System that stores more than 1 kWh of power. Code 706.3 absolutely requires these to be installed by a "qualified person", so unless you are a licensed electrician, licensed solar installer, trained lineman, manufacturer trained technician, or an Electrical Engineer (or have been trained by and are working under the supervision of one of those) it is unlikely the AHJ would consider you a qualified person (and they may not consider all those listed as qualified, either, those are just people they might considered qualified), making the installation a code violation, even despite the normal homeowner electrical exception.

This device has a metal case with a grounding screw, has documentation that describes no internal bonding to neutral, but indicates that this is required externally. It also indicates the existence of an isolation transformer. Thus we have a Grounded Separately Derived System.

If this were not a ESS, then yes, a main panel with the bonding jumper would be correct. However, because it is an ESS, under NEC 2020, it will need an output disconnect that can be locked open within line of sight of the inverter (706.15). If this disconnect were not outdoors, then it would need an additional outdoor disconnect if this is a single or two family residence (to act as an an emergency disconnect). Since the inverter is not intended for installation in freezing environments, even an outdoor rated main panel with a main breaker that can be locked in the off position may not work, so you likely need a sperate disconnect (or two) and panel wired like a subpanel.

If later being used as a solar inverter there are a slew of requirements for that as well that will need to be met.

The very first disconnect outside this device will need to be rated for use as service equipment, and connected like service equipment (i.e. like a main panel, except for not having spaces for breakers.) Specifically:

  1. You must leave have a System Bonding jumper in place in this first disconnect (per 250.30(A)(1)), which will need to connect the neutral, the Grounding Electrode Conductor, and the Supply-Side Bonding Jumper.
  2. You connect your disconnect to your grounding electrodes via your grounding electrode conductors, much like you were installing a second electrical service in the same building. You must use your existing grounding electrode system (as per 250.30(A)(4)), and connect it to your disconnect via a Grounding Electrode Conductor like always (as per 250.30(A)(5)).
  3. You must have a Supply-Side Bonding Jumper that connects to the grounding pin in the corner of this inverter (as per 250.30(A)(2)). Code provides the sizing details. I'm not sure if using conduit or other metal raceway as the supply side bonding jumper is actually feasible with this inverter.

Obviously after this initial disconnect, the feeder will be 4 conductors including an Equipment Grounding Conductor. As discussed it might need to travel to a second disconnect, but would otherwise head to the new panel set up like a subpanel.

Plus there are a variety of specific energy storage system labeling and other requirement I have not discussed yet.

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