A commercial water heater can be quite different from the water heater that is in your home. The most common size for a commercial water heater is between 75-100 gallons. There is a wide range of demand that may be needed for a commercial water heating application. 

Commercial water heaters will vary greatly in price mainly due to the output and energy specifications. A water heater that has a quick recovery system can easily outproduce a much larger tank. For instance some 80 gallon water heaters will  significantly outperform other 100 gallon tanks. The price difference can also vary several thousand dollars. A 100 gallon commercial water heater can start out at $1800 and can go up to $7000, some are even higher.