Water heater size depends on the number of bathrooms and bedrooms in a residence. A two bedroom home with one and a half bathrooms requires a 30 gallon water heater.
An undersized water heater will have to work harder and therefore will have a shorter lifespan. Be sure that the water heater capacity will fit your home needs. For example, a five bedroom home with two and a half bathrooms requires a 60 gallon water heater. A chart to determine your specific needs is available on HomeDepot.com under the Buying Guides tab. The Water Heaters section can be found under Heating & Cooling.