[nerdbert]

Until a couple years ago we had a gas-fired tankless water heater which used a single D-cell battery for ignition, which had to be changed once every year or two. It wasn't connected to wall power at all.

[hinkley]

But you need a blower or pump in order to circulate heat in a house. Hot water is better than nothing but it won't keep your fish from freezing.

[alvah]

There are, or used to be, Bosch tankless water heaters which used a small turbine connected to a piezo-electric igniter, no external power source required ever.