Hot water heaters come in two primary configurations, a tank water heater or tankless configurations. Either configuration comes in a variety of fuel types such as natural gas, electric, and propane. A tank water heater is comprised of a water storage tank, anywhere for 30 gallons to 80 gallons, and a heat source that heats the water. Conversely, a tankless unit does not have a tack and heats the water instantly based upon the demand for hot water. Each has its pros and cons and the type of unit you need is more dictated by install location and personal preference.
Traditional water heaters have a tank to hold water with a heating element inside to heat the water. The heating fuel can be electric, gas, or propane for conventional tank systems. Tank water heaters can range in size from as small as 30 gallons to over 100 gallons. The size of the tank depends on the demand, the more people there are in the house, the larger tank you will need. Many advancements have been made with tank water heaters, some new water heaters are made with complete carbon fiber tanks and are guarantee to never leak.
A newer type of water heater that has been making its way into the U.S. market is the tank-less water heater. Used extensively throughout Europe, tank-less water heaters offer unlimited hot water and substantial space savings over their "tanked" counterparts. Additionally, they are more energy efficient because they only heat the water when needed as oppose to continually heating a tank full of water.
With either water heater, cost and sizing are the largest consideration. It is imperative to size the generator based on the current and projected hot water demand. The usage of two adults versus two adults with kids is substantially different. Cost is always a driving factor with any hot water heater purchase and the prices vary as much as there are options.
The general rule of thumb for replacing a hot water heater is between 8-12 years. Water heaters are typically not repairable appliances. That is not to say it cannot be done, but, typically if a water heater starts to fail it is best to replace the unit. Especially if it is beyond the manufactures warranty period which is usually 10 years.