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.