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I'd like to put some solar panels near my house and power a small sub panel with a few key circuits. Like the fridge and freezer. The idea is to reduce my electrical bill, and have some power to use for those circuits during power outages.

I live in Washington, US, and we are using the NEC 2020 code. I won't be sending power back to the grid. All the UL1741 certified inverters I can find are either inefficient, or have really bad THD. It's been frustrating. The DIY'ers out on the internet seem to be installing this stuff with no regard to code.

Ideally I'd like a hybrid off grid inverter that could manage power from solar/battery/generator and send it to the sub panel. I know that grid-interactive inverters are required to have a UL1741 listing. But what about off grid inverters used in a house? Do they need that too?

Follow-up question: So what kind of stamp of approval (if any) do I need to pass an electrical inspection if the inverter is permanently installed in my house and not an RV?

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    "still" :) I realize there's a certain amount of Washington pride for being almost first to adopt NEC 2020... but most Americans are still on 2017, and only six have even started evaluating 2023 for possible adoption. According to NFPA anyway. Commented Apr 13, 2023 at 23:03
  • RV (standalone only) inverters are listed to UL458 -- it's only inverters capable of grid-tied operation that require UL1741 listing. Is there a reason you don't want to send power back to the grid ever under any circumstance, by the way? Commented Apr 14, 2023 at 2:49
  • So would code allow a UL458 inverter to be permanently installed in my house?
    – Mike Gray
    Commented Apr 14, 2023 at 19:55
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    @MikeGray -- provided it isn't trying to push power to the grid ever, sure! Commented Apr 15, 2023 at 3:28

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OK you're a bit confused but straightening that up is what we do :)

Any equipment you install in home wiring needs to be approved (NEC 110.2) and that means by UL, CSA, ETL or other "NRTL". It must be designed for the purpose you're putting it to, and you must use it according to its labeling and instructions (NEC 110.3).

While UL is a testing lab, they are also a standards-writing organization. They write thousands of product standards, such as the UL White Book and numbered standards.

There are 2 different kinds of inverters.

Generally, they break into "Standalone" and Grid-tie". Though there are combo inverters that do both.

Grid-tie inverters are for people who live "on the grid" and exclusively want to sell power back to the utility while the grid is up. They must interact properly with the grid, shut down when the grid is down (anti-"islanding" to avoid zapping linemen), and not start when a house is on generator. The standards to do all that are UL 1741. But this is not the inverter you want.

You want an off-grid or grid-down inverter, aka "plain old inverter". Those follow UL 458 for their product standard, though you don't really need to know about that - you just need to select one that is UL, CSA or ETL listed for your purpose, and use it per instructions.

That third type is a grid-forming designed to do both in one box, while interlocked with an isolation switch so it will only "create a grid" if the switch has been pulled. (on Tesla Powerwall and other serious grid-forming kit, the switch is automatic). These emulate the grid so well that they fool UL 1741 solar inverters, and they use the generated power to recharge their battery.

I wrote that last paragraph because I want you to know that technology exists, to distinguish it from what you want. It's certainly a waste of money for you.

Your "critical loads subpanel" can be a simple thing

We recommend large panels because breaker spaces are cheap and running out of spaces is annoying and expensive. But what you need is dog simple: either

  • any panel with a main breaker, for which the manufacturer offers an inexpensive sliding-plate interlock. And a breaker.
  • A Siemens 12-space or larger panel with two breakers, interlocked by the $30 ECSBPK01 interlock. This is a slick solution because the interlock provides the tie-down kits for the breakers and complies with Canada code regarding the interlock working with the cover removed. It's also more aesthetically obvious how it works. You can do the same trick with an Eaton CHML or Square D QO2DTI interlock.
  • Or, simply use the main panel you already have, and put a sliding-plate generator interlock in that if it's offered. That will throw the whole panel over. Downside: can't leave a utility-side light turned on so you won't know when power returns.

From your regular panel you have feeder wire (we like 2-2-2-4 SER aluminum for 90A; it's cheap) from the utility panel to the critical loads subpanel.

Tricks to wiring up the critical loads sub

When you install the critical loads sub, install at least five EMT conduits between main and sub. One large one for the feeder, and four small ones for various circuits you plan to bring over. 1/2" will suffice but the knockouts are 3/4" so might as well use 3/4". If your flag does not have a red maple leaf, it is legal to leave the existing circuit's Romex wiring in the existing panel, and bring over the hot and neutral in THHN wire through the conduits, to land it on a breaker in the critical loads sub. If the conduits are over 2' long you are limited to 4 circuits per conduit.

EMT conduit is a valid ground path, so five EMT conduits is gross overkill for grounding requirements, making a ground wire unnecessary.

Note that MWBCs (Multi-Wire Branch Circuits) aka "2 hots 1 neutral" shared-neutral circuits, cannot come over in parts. The entire circuit must come over, or not at all, and it must be on a handle-tied double breaker with 240V across it. MWBCs cannot be powered from a 120V subpanel, as this will overload the neutral.

Now, let's talk about that inverter and the B word.

Once the critical-loads sub is finished, all you need is an off-grid solar inverter that takes power from solar panels. Wire that up to the "generator" backfeed backer in the critical-loads sub (or the generator breaker in the main panel if you went with a main-panel interlock).

It should be UL listed or equivalent, and install it according to labeling and instructions.

Literally everyone who has ever asked this question has aspirations for a battery-free system, but I don't think that's realistic, and in any case, makes implementation a lot harder. Of course demand for batteryless systems is high, so maybe the marketplace has come up with something new. Go see!

Inverters need batteries because solar is not reliable/stable, and it can't handle a surge/startup load: when a refrigerator or furnace motor presents 2-3kW of instantaneous "Locked Rotor Amerage" startup load, that electricity simply does not exist because you don't have 3kW of solar production right now. So it doesn't happen and the inverter shuts down. Whereas with any size of battery in the system, this isn't even a speed bump. The battery takes up the slack.

If you can settle in on the idea of batteries, this is very straightforward. Consider alternative batteries other than boring old lead-acid, which doesn't age well. Many people either build lithium battery packs with 18650s, nickel strips and spot welders... or they up-cycle EV batteries out of crashed EVs. For instance the 250 watt-hour Nissan Leaf LiPo modules for $40, or the 7000 watt-hour Tesla Model S L-Ion modules for $1000. At this point EV batteries from wrecks are cheaper than lead-acid when you consider usable range. And EV grade batteries are higher quality than 18650s, and likely to have a much longer life.

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  • Hadn't realized that uoctcled batteries were that affordable... Note that this restricts your choice of other solar equipment, though; a system such as mine with mini-inverters per panel doesn't have a good place to patch in a DC charge/discharge system and would have the overhead of converting AC back to DC in addition to the inverter to produce AC from the battery's output. .. I wonder if anyone is selling what amounts to a high-power UPS box for that.
    – keshlam
    Commented Apr 14, 2023 at 12:54
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    Thanks for all the info. But to get back to the original question, what sort of certification would the inverter need to pass an electrical inspection?
    – Mike Gray
    Commented Apr 14, 2023 at 20:00
  • @keshlam it's better than that... a Nissan dealer parts department will sell you a brand new 62 kWH Nissan Leaf pack, no core, for $15,500. A quarter a watt-hour from a dealer. On your solar, "Rapid Disconnect" rules require that firemen be able to push a button and segment your solar array into safe voltages <30V between sections. The market response was what you have, microinverters: pull the meter and they all go dead (per UL 1741). "But that makes them useless in a grid-down situation" <- solution: the Grid Forming Inverter tricks the UL 1741 inverters into coming back up. Commented Apr 14, 2023 at 22:50
  • If grid is down, I'm probably content to run on battery for a while; island mode would be better, to stretch the battery time, but isn't a necessity. But, yeah, going offtopic.
    – keshlam
    Commented Apr 14, 2023 at 23:35
  • @Mike I edited yesterday to add that. It's in the first part. Commented Apr 15, 2023 at 22:31

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