Either you will have to install a larger gas or electric supply, or the unit will have a reduced heating rate which will limit the amount of water youwill have available. The units are only suggested when you have widely separated usage areas and install one at each area. They are not inexpensive, and repairs are expensive.: I have to replace my hot water heater and I wanted any information you have on instant hot water heaters. My hot water heater size is 40 gallons.