Propane and natural gas are the two cheapest sources of energy available for home use. When you combine gas with an efficient water heater such as a tankless unit, you basically get double efficiency. With a tankless gas water heater at your disposal, hot water is a steal.

Heat pumps or hybrid water heaters as they are often named are systems that give a larger heat output than their maximum energy input. They do that by collecting the heat in the air and channel it to well-insulated hot water tank.