Do not forget to think about water hardness. I live in a hard water area and change gas heaters for 80Ltr hot water electric boiler as guests in our rental property could not handle the gas instant heater and were wasting water. Now we have the problem of scale on the element and carrying out maintenance to remove. Yes we have in line scale inhibitors and still trying to get them right. The best is instantaneous gas water heaters as you use it. In the states there are not many options and expensive compared to here in Spain.