Conventional water heaters are by far the most common type of water heater in the U.S. today. They range in size from 20 to 80 gallons (or larger) and are fueled by electricity, natural gas, propane, or oil. Called ‘storage’ units, these water heaters transfer heat from a burner or coil to water in an insulated tank. The down side to a conventional water heater is that energy is consumed even when no hot water is being used.
Tankless water heaters do not contain a storage tank like conventional water heaters. A gas burner or electric element heats the water only when there is a demand for hot water.    _{Your Comment}