The traditional water heaters in that have been in American households for many years are the typical the storage tank models. In recent years when replacing failing units, many consumers have been switching to the more energy efficient and eco-friendly tankless water heaters. The problem with the "old fashioned" hot water tanks is that there is a significant amount of wasted energy when it is not being used - referred to as "standby heat loss." Having to keep the water in the holding tank hot until ready to use is a big waste of energy.
There are things to consider though before you rush to buy a tankless heater. Doing your research and knowing their capabilities will keep you from running short of hot water. Although manufacturers like to brag about the "endless supply of hot water," tankless hot water heaters can only realistically put out a certain volume of hot water at one time. The amount of hot water they are capable of delivering is based on their size and flow rate. Flow rate is the term for the measure of water in gallons per minute (gpm) that the unit is capable of putting out.
In order to determine the size and type of tankless heater you need, the size of the household, along with estimated demands on appliances that may run at the same time as taps will be used will need to be taken into consideration. Whenever you exceed the limits of any system's flow rate, you will not have enough hot water. In some cases, the solution may be as simple as adjusting your schedule so that you do not run too many appliances at one time. There are other solutions to the problem if modifying your routine is not an option.
Installing a point-of-use water heater would be one of those solutions. A point-of-use water heater is installed right at the water line supplying a particular appliance, a dishwasher for example, and it will only heat the water flowing directly into the machine. They are much less expensive than whole-house water heaters and appropriate where demand is at its greatest.