In most homes, the most common appliance for heating water is a large tank, which is then heated using natural gas or electricity. The hot water is stored in a large container, which itself might weight a few hundred pounds even when empty. The hulking unit for many homes may be a blessing, because it ensures a large supply or hot water when it is needed, and resigns the task of heating that water to a gradual process that saves energy. Water has high thermal mass, meaning it takes a lot of energy to heat it up, and it stays hot in the tank for a long time. Think of the persistence of a hot water bottle. If the power went out, you would still have hot water for at least twenty-four hours.
Traditional heating units have their drawbacks, too, and one of the most significant is that they supply heat gradually rather than on demand. Imagine if you ever use more water than the contents of your tank, such as many people bathing. Not everyone enjoys a cold shower, and even more dislike a luke-warm shower. There is such a thing as an electric tankless wa ter heater. Electricity never runs out, and so it is not possible to run out of hot water, not unless you draw too much for it to heat efficiently. If you have an ample supply of electricity, then you can have a tankless electric water heater.
Most homes are connected to the grid, so this is not a problem. Electric tankless hot water heater s can be found, however, on mobile units such as camping trailers. The units are noticeably smaller, supply a smaller stream of warm water, and run on direct currents, but they are the equipment of choice for campers. It is faster and more efficient than trying to maintain a tank in limited space. The tank is for cold water, and it is certainly more efficient not to have a hot water tank when it is used infrequently, and especially when the power is off because the vehicle is parked.
In the home, instant water heaters are a backup in case too much water is drawn from the tank. It can also be the primary heating unit in situations like a remote home that runs off an array of batteries. In these cases, the water heater may have been salvaged from a camper, or bought from supply store that normally caters to campers and their repair.