If the grid is down, though, then the subpanel critical load breaker being 70a would be well above the threshold to overcurrent protect the inverter output anyway.
And this is also true in a 'whole home backup' scenario that permits the 200a downstream load.. without grid support the inverter cannot rely on the load breaker to protect itself, that would be insane
Just keep in mind, NEC is not about protecting the inverter output, you are protecting the wires and connectors that feed the services. Thus you can come out of your 200A panel, with a 15A breaker connected to #14 wire to a 5-15 socket, and plug a 120v/2A wall wart, which has a 2A fuse that protects the #18 wire from cooking. The breaker does not protect the load, it protects the wiring from the load if it goes south, as Tim states above. If you start backfeeding power it gets wonky because the wiring (the bus bar) was engineered with specific ratings for voltage and current, the latter which could be exceeded with multiple feeds to the bus bar, since the sum of the breakers in the panel generally exceeds the panel maximum. NEC could care less if your inverter falls over as long as all the wires and connectors feeding the loads are protected by appropriately sized breaker for those wires and connectors.
All this is a little esoteric to me. Just isolate your sub-panel to the load lugs on the inverter, thus it only feeds off the inverter, and make sure the wires are suitably sized to the panel breaker. I try to not over-complicate this stuff, If you confuse yourself it will probably confuse the inspector.... I see a box, it has 2AWG that runs as input to this panel with a 100A breaker. Check. If your gonna grid tie, you get to do the 120 math, but I always thought the simplest and safest thing would be to drop your (200A?) primary feed directly to the inverter/grid bus, (via a fusebox or small panel) and run everything from there to your load panels. Then I can backfeed whatever and never overload the buses past the breaker ratings. When i started looking at this weird backfeed stuff with 70A breakers, and "Solar Ready" panels, my first thought was about overloading the bus, then I realized this is useful for a micro-inverter setup that is supplementing the grid where all your solar power just gets dumped into your main panel, and absorbed by whatever loads from there.
When you start wiring up an inverter like the 18kpv/12lv or the big Sol-Ark, you can do it exactly the same way, and just ignore the load lugs, now you don't have any backup. The minute you decide to mix output with some from the inverter and some from the grid and multiple panels it get's complex. Like ICE car, Hybrid car, then EV, if you go ahead and build out your inverter setup to handle all your loads, you re-simplify and use the grid as a backup for your PV setup, at which point you should be able to safely back-feed up to the feed breaker rating if the power company will let you. Your mileage will vary, but to me the real benefit starts when my setup actually can run my whole house.
