Heating water is a huge energy draw. It's something I've struggled with for years.
When we first moved in to our place in the woods, propane heated out water. Our home was one of the first in the area to have a tankless water heater. When it worked, it worked great. I just turned a faucet on, the propane burner ignited, and it made hot water as fast as you could use it. As long as there was propane in the tank and water in the well, it would make hot water.
My lovely wife and I raised three girls. My girls loved their hot showers. Tankless water heaters do the job, and do it well.
I don't have one anymore.
Our tankless water heater was made by a company called Myson. It was directly vented to the outside -no chimney or power vent needed. Also didn't need electricity. (at first) It worked great until the weather turned cold. It froze solid. The local propane dealer did not know the heater needed something called a freeze kit. The freeze kit had a little electric warmer plate that was supposed to keep everything from freezing. So much for not needing electricity.
Sometimes it worked. Sometimes it didn't. A steady procession of propane repair men marched through our basement. My kids learned all their names and began to think of them as extended family. Valves, diaphragms, burners, and even a heater core were replaced. This went on for 10 years until the warranty expired. The propane company out and out refused to put in a new tankless water heater for me. Being without hot water, and with three teenage girls and a wife waiting for hot water, my negotiation position was very weak. I soon caved in and let them put in a regular propane tank water heater.
It worked well enough, but wasn't anywhere near as efficient as the tankless. Soon after, the girls grew up and went their separate ways -lowering hot water usage quite a bit. However, the price of propane keep climbing and it became more and more expensive to heat water that way.
One winter we came home from our southern travels and reopened our house. The propane burner, for reasons unknown, had failed. The trip had cost a lot more than planned as we to pay for extensive vehicle repairs. I wasn't going to just replace one propane water tank with another -funds were tight.
The local building supply store was running an unbelievable sale on small electric water heaters. I picked up a little 4 gallon tank and plumbed it in place of the 40 gallon propane. It was a start, but we wanted more hot water.
Even though the propane part of the water heater failed, the tank itself was still sound. I stripped all the propane fixtures and the insulation off the heater. Painted the tank black and enclosed it in a box made from salvaged windows. It became a solar batch water heater. The home brewed solar heater dumped into the 4 gallon electric tank.
If the sun was bright, the little electric tank didn't have to work at all. Even on fairly cloudy days, the batch heater would bring the temperate up some. The addition of the small electric heater, combined with solar heat, barely increased electric usage.
Spring, summer and well into the fall, it worked pretty well. Of course, the batch heater would freeze come late fall/early winter. Before then, it was disconnected and drained. Knowing this would eventually happen, it was all hooked up with screw on connectors that were easy to remove.
Then I was back to just 4 gallons of hot water. I can take a show with 4 gallons. My wife on the other hand . . .
In the interests of domestic harmony, I set out to get more hot water. My original idea was to buy a special tank to hook up to the woodstove. Those cost real money. However, electric water tanks the same size are pretty cheap.
It's just a matter of building a heat loop into the system. Hook a line into the tank drain. That's the cold return of the loop. Then it goes up to a copper coil that sets on the back of my woodstove. The heat from the stove causes the water to circulate. From the stove it goes up and dumps into the cold water feed of the water tank. Works well enough.
Since the tank still had its electric coils, I figured I might as well hook those up too. To heat water electrically, just flip a switch on the wall. That activates a contactor that powers the coils. We only use that feature once in a while, usually when we have company and go though more hot water than normal.
It's possible heat water completely without the grid. The batch solar water heater could be moved to a sunnier location. The 4 gallon water heater (solar electric connected this time) could be hooked up on a switch -to be used only when a little extra boost is needed for the solar. For the winter, building a slightly bigger coil for the woodstove should do the trick and make enough hot water.
Water temperature would vary more than what people expect. Still, it'd work and be renewable. Might just try that come spring.
Then there is the quick and cheap water heating method. Got one of those black shower bags used for camping. Put it in the sun during the summer. In the winter, keep it by the woodstove. Worked for a bachelor friend of mine. Of course, he was a bachelor. I wouldn't dream of trying that trick with my lovely wife.
Large hot water tanks are much cheaper than a divorce.