In the winter I run my wood stove with a hot water coil in the firebox. It's connected to a old 60 gal solar hot water heater. It is connected to the electric hot water heater in such way with ball valves so I can run on electric only, wood stove only or I can preheat the water going into the electric water heater.
By the time it's cold out and I am burning wood full time I shut off the breaker to the electric water heater and bypass it with the ball valves. I save about $15 a month by this alone while the water heater is off.
I would like to get a electric tankless heater. I could hook it up to the solar (wood) heater and have the best of both worlds. The tankless one should only have to heat the water to 120 in the summer and wouldn't have to turn on much, if any in the winter. Plus I could eliminate all the valves.
They use electric tankless water heaters in radiant heating all the time...whats the difference really?