Does your water bill show you gallons used/month? Ours does.
Then you need to know the power ratings of the units, i.e., the voltage they work at and the max amps they draw.
Find the typical well depth in your area. Assuming you're in the USA, there may be a well registry for your state. Arizona has one. I have looked up my neighbors wells.
There are alternatives to instant hot water like pre-heat systems that circulate the cold water in the pipes back to the tank and then fill the pipes with hot water. it takes a few minutes, but it eliminates water waste.
I really want to discourage you from going tankless electric water heater off-grid. They are horribly inefficient and demanding. When I said it doubles your system size, it might even be triple. A heat pump water heater is 4-5X more efficient (uses 1/5-1/4th the electricity to heat water vs. traditional hot water heater or tankless), and you can get the same water savings with a pre-heat system.
