Water heater ratings are done on the basis of the performance and built up of the system, and its utility of energy. There are water heaters with various models that operate either on electricity or gas. Now, there are even propane based water heaters. The water heaters are available in both the models; with tank and without tank. There are water heaters based on the hydronic technology, they are called central water heating systems. Among all these, the tank less water heaters are rated as the best, due to many reasons:

* The tank less water heaters don't have boiler. They use the technology of instant heating. Tank less heaters are more efficient in comparison to storage water boilers.
* Tank less water heaters can use both types of installation, the centralized as well as the POU mode.
* The absence of a boiler saves a recognizable amount of energy, unlike the conventional heater, that has to reheat the waters in the tank when it cools off.
* In a general central water heating system, much of water is expunged when the cold water in the pipes is allowed to fall off, waiting for the hot water to run.
* There are gas water heaters, that use either home boiler or the furnace as the source of heat.
* In conventional heaters with boilers, the water turns cold when the electric connection is unplugged.