Manufacturers list the water heater capacity on the ratings plate. With tank-style water heaters, the capacity is in gallons. This plate includes other important information, including the manufacturer, model, serial number, wattage of the heating elements and length of the warranty.
Knowing the capacity of the current water heater helps the homeowner make decisions regarding a replacement. If the family often depletes the hot water supply when using the current unit, the owner should consider a larger capacity. However, larger units require more energy to operate.
The capacity of tankless water heaters is provided as the maximum temperature rise possible at a given flow rate. With these models, the unit does not store hot water in the traditional tank but heats it on demand.