Why do you think it takes a lot of energy to keep the battery in a usable state? It’ll take quite a lot of energy to get to that state.. but the energy it takes

Why do you think it takes a lot of energy to keep the battery in a usable state? It’ll take quite a lot of energy to get to that state.. but the energy it takes to keep it there is not just a function of the temperature. It’s a function of how much insulation you have as well. Insulation is cheap and it gets easier/cheaper the bigger you scale it. Keeping something at hundreds of degrees for a long time without adding energy is not a very hard problem. There are companies that are making seasonal energy storage for colder climates using this very mechanism. Just heat something up in the summer, and extract the heat in winter. E.g., see Polar Sands thermal battery. They warm sand to 600°C and it can stay hot for months.