Tankless gas water heaters have been around for a long time, well over a decade now. Sounds like you don't know very much. Tankless electric water heaters do require a higher amperage. Mine had three two-pole 40A breakers. I had to upgrade the service to the old house I bought to 200A before I could install it. The amount of energy required to heat the water is the same though, and actually tankless is more efficient because you aren't paying to keep the water at temperature when not using it. Also, you may have had hot water 20 minutes after the power shut off, but I bet you didn't have hot water for a long time after the power came back on because it takes time to heat up a tank full of water. You can also run out of hot water with a tank heater, but not with tankless.