If you are in the store for a new model, there are two kinds to consider: a storage tank model and tankless water heater. A storage tank model keeps hot water in a tank until it is needed, whilst a tankless version heats it up on demand. Whereas each has its feature and Disadvantages, the storage tank varieties are more common. Count on the size of your house and family, you may require a small or large unit. Usually, the small family will only need a model that holds little gallons; but big family will need more gallons unit. The size of your storage area also defines in the size and kind that will work preferable for your house.