Water heaters typically fall into one of two categories, the tankless water heater or the tank-style heater. The tank-style heater usually can keep anywhere from 5-gallons up to 120-gallons heated and ready for instant use. The tankless water heater is designed to conserve energy by heating water on demand as required. Tankless water heater sizes also vary, but they typically take up less room as they are not required to hold large quantities of water. Plumbers and home builders are often tasked with the installation of water heaters.