This is an interesting video produced by Rinnai that talks about how a tankless works. They go over the very way to address the amount of water that a tank produces verses that of a tankless. They also talk about the lifespan of a tankless compared to that of a conventional tank.
With a tank water is continually heated and reheated wasting valuable energy when hot water is not in use. On top of that your hot water supply is limited to the gallons of water your tank can hold. A Rinnai tankless water heater is more efficient, providing endless hot water only when you need it. A Rinnai tankless has double the lifespan of a tank, making it ultimately the better value system.