If you scale it up it can probably deal with a week. People have pretty bad intuitions about heat loss for big objects because most hot things in our lives are

If you scale it up it can probably deal with a week. People have pretty bad intuitions about heat loss for big objects because most hot things in our lives are small, but since heat loss follows a square cube law, every 8x volume increase gives 2x decrease in lost heat. It's also important to remember that this doesn't need to be anywhere near 100% efficient. Given the swings in energy prices, you can probably make a profit at 50% efficiency.