When it comes to residential hot water tanks there are multiple different options such as electric, gas (Natural Gas/Propane), on demand hot water or hybrid water heaters. The size of the water heater depends on the load it is required to carry in the home.
Electric – Electric water heaters are what most people think of when talking about water heaters. They are easy to maintain, require no combustibles or venting and offer high energy factor ratings.
Gas – These systems normally require a slightly larger up-front investment, but usually cost less to operate. They must be vented outdoors for safety. The bonus to these units is that some are not affected by power outages.
On Demand – These units are typically operated by gas, they are very small and in most cases wall mountable and so take up much less room than a normal tank. They are also much more efficient than a normal tank that maintains water temperature, these units produce hot water only when it is needed.
Hybrid – Hybrid water heaters are basically a water heater with a heat pump attached. Using the same technology as a heat pump these units heat water using the heat from the air around them. They are extremely efficient and reliable systems.