Here is a water heater that produces hot water on demand. The Essency EXR has a 55-gallon footprint. The water inside acts as heat storage, so when you draw hot water, regular water is run through the heat exchanger to heat up. Once you fill the tank, the water stays in there as long as you use the heater. The tank has a polymer composite construction.
The Essency EXR Water Heater uses standard plumbing and electrical connections. It comes with WiFi connectivity. You can find out more about it here.