I think it's all about scalability where I'd imagine there are multiples more Tesla battery packs produced and sold for vehicle use that for structure power backup and so the price is accordingly. There are several YouTube videos of folks building their own residential power storage using recycled Tesla modules, lead-acid batteries, and even discarded laptop batteries. Believe it or not but there's even a Republican congressman who has a series on YouTube about his journey from upgrading his lead-acid battery bank to Tesla modules.
I hope to build out a backup battery bank for the house and one day an electric-sailing catamaran. Someone did that with a used 40ft Lagoon using OceanVolt servo props down in San Diego and the boat is available for charter. According from what I read, the batteries are recharged by shore power/solar power (trickle)/hydro-generation. Similar to our cars regeneration, the boat regenerates under sail with a ratio of 3:1. 3 hours of sailing with >8kts of wind will generate enough electricity to operate the electric motors for 1 hour. That's a pretty amazing ratio.
Experience the quiet of electric propulsion, and the charge of sunlight, on our Leopard 40 Catamaran, Ramblin' Rose by renting our charter through Seaforthboatrental.com, in Coronado, CA.
www.sunwatermarine.com