Question:   What is a tankless water heater?
Answer:    A tankless water heater is a water heater with no storage tank that is activated only when there is a demand for hot water. Therefore using energy only when hot water is needed.
A tank heater consumes energy 24 hours a day whether you use it or not, constantly turning on to keep the water in the tank at the desired temperature.
Tankless water heaters also provide unlimited hot water because there is no stored capacity to run out.
A Tank water heater will run out of hot water when the stored capacity is depleted and will not be able to provide more hot water at the source until the entire tank of water is reheated.
Because there is no storage in a tankless heater, the physical size is much smaller allowing for many more installation option than with a tank heater.