I'm in the middle of a renovation now, that will include batteries and solar.

One thing that I never see mentioned is how to support on-demand instant hot water heaters.

The 6.5 gal/min heater (what it takes to fill a tub) that I'm installing uses 100A at 220v when operating!

I haven't found any battery system that can support that.

Do "totally off-grid houses" all use a typical electric storage tank hot water heater? For my solo occupation, that's a lot of hot water storage over times when almost none is being used.

I do have a smaller water heater under the kitchen sink, so that the giant one doesn't have to run for that usage.

How do off-grid homes deal with the high instantaneous current consumption of on-demand water heaters?

They don't. You either have a solarthermal system to heat the water, or you use a tank to store hot water. I've previously installed an "iBoost" which diverted excess solar into an immersion heater.

Heating slow(er) and storing is going to be easier than suddenly ramping up for a shower.