Got it. So I wasn't misunderstanding you and you do have the water heater connected to the grid. It doesn't matter how many switches, disconnects, GFCIs, or breakers you put between the panel and the heater, it's still connected to the grid,

So, how about this idea: Install a plug on the water heater wiring harness, install a 20A 240V outlet on the wall of the water heater closet, and plug the water heater in? I could use this receptacle:

https://www.amazon.com/Leviton-5461-R-Industrial-Receptacle-Grounding/dp/B000HEFBNA/ref=asc_df_B003GIS46K

And this plug:

https://www.amazon.com/Leviton-620PV-Volt-Grounding-Yellow/dp/B000U3HYQK

When I want to run off solar, I would unplug the water heater from the wall, would that mean it's no longer connected to the grid? My inverter isn't connected in any way other than through house lumber to my service panel or any part of my house's wiring. Would that be NEC compliant? I have the 2017 here, my AHJ has adopted the 2021, but I doubt there will be any changes in the section(s) that would cover this.