[Ooper]

There is a long payback period for tankless heaters, but what you are paying for is not economical payback... you are paying for convenience... the convenience to have relatively quick hot water at the point of usage since they can be installed in closets or under sinks. You do not have to wait for the hot water to travel from the garage to the opposite end of the house. This is assuming you install the heater near the point of usage and not in the garage. Installing it in the garage is really self defeating the purpose.