Natural gas and propane are two very similar fuels which are gaseous at normal ambient temperatures. They are both readily available and clean-burning fuels for tankless hot water heaters and other appliances. For gas tankless water heaters, typically, natural gas would only be an option for your home or other building where it is already available (piped in by your local utility company).
Propane is also called LP gas or sometimes simply LPG. The L stands for liquefied, indicating that propane can be fairly easily liquefied and stored by putting it under moderate pressure. Liquefying the gas causes it to require much smaller storage containers. Conversely, natural gas requires much higher pressures to liquefy it, so it is normally stored and transported as compressed gas (which requires considerably more storage volume per unit heat). The availability and convenient storage is why LPG is a popular fuel for home BBQs.
So what would cause you to choose one or the other for your tankless water heater?
Pros and Cons of Natural Gas Tankless water heater
Pros and Cons of Propane Gas Tankless water heater
Popular makers of propane tankless hot water heaters include Rheem, Bosch, Rinnai, Eccotemp and others and generally are very similar to the same manufacturer’s natural gas units. Reviews for tankless propane water heaters are largely very similar to the corresponding natural gas tankless heaters.