A hot water tank is a storage water heater, that is, hot water is stored in a container at your home. Using hot water, you empty the tank which immediately fills with cold water. If you consume massive amounts of water in a very short period of time, there may be a shortage, and it will take a while for the stock to rebuild. The second type of water heater is the instantaneous water heater, which continuously distributes hot water. It does not have a storage tank, which saves considerable space. However, the water flow is much lower. This is why it is rather used as a backup water heater, while the hot water tank is quite suitable as the main source.