Price out a 1000 Ah 48V battery.
Then price out a tank-type water heater, and compare them.
Battery backup is nice. Backing up an instant water heater is absurdly expensive. Backing up your communications gear and a furnace fan or electric blanket is more reasonable. Sufficient PV to power A/C while the sun shines, coupled with a small battery, is also reasonable. That is what I have.
Something like SolArk, with tank-type water heater connected as a dump load (part of zero-export) could meet your needs. Somebody else can help you specify and configure that, I'm a 100% SMA shop.