i looked into this kind of setup when i needed a new hot water heater years ago.
at that time, i was advised that for my home use, it was better to buy a conventional water heater because of the cost to payback ratio of the on-demand units just wasn't practical, and there were a couple of other things i didn't think of at the time, but i forget what they were.
i don't know if anything has changed over the years, but at the time i needed one, my local plumber advised against it.