Hi John: From the numbers, you must be selling Takagi. I've looked at their spec's and I'm very impressed. They are apparently the most efficient tankless heaters on the market. I think I'll be adding them to my line. Good luck John. For what it's worth: There is never a full tank of hot water with the same temp from top to bottom. This is called temperature stratification, and as soon as you start to draw hot water off, the tank starts to fill with cold water, which blends with the hot, which cools the volume down. Hence, there is NEVER a full tank of hot water. As I said, and john agrees, the only way you can put 50 gals of cold water in, and get 50 gals of hot water out, is to heat it with a tankless water heater. It's the only way you can do it. In my humble opinion. Keep up the good work John, I hope you have lots of happy customers. I do. Brian Wood.