May I ask what brand? Since we are on well water and pump we would have no water if it wasn’t for our generator.
Run a metal water pipe through back or on bottom of wood stove before it goes into water heater. It’s actually more economic than the tankless
Mother Earth has a couple of article on how to do it. Also in the foxfire books iirc
Lysie our tankless water heater is a NAVIEN. We can have the washer and two showers all going at the same time, and have never run out of hot water.