Hot water needs: In general, your hot water needs can be measured by the number of bathrooms in your house. The smallest size unit for a one-bathroom house should be 30 or 40 gallons, in either gas or electric. For a 1 1/2-bath house, 40 gallons is minimum. For a 2- to 3 1/2-bath house, choose a 50-gallon gas heater or a 66- to 80-gallon electric one.
Electric considerations: Electric water heaters take longer to heat water, so large tanks should be bigger than their gas-fired counterparts. For a large, four-bath house or a home with an extra-large bathtub, get a 75-gallon gas heater or a 120-gallon electric heater. Some circumstances can skew these standards for example, a laundry-heavy family with small children or a house with an especially large bathtub will mean much higher hot water use than normal.
Metrics: A typical load of laundry uses 22 gallons of mixed water. The amount of hot water depends on the temperature you choose. A typical bath uses 43 gallons of mixed water. The amount of hot water depends on the temperature you choose.
Heater sizes: Water heaters come in three basic sizes for homes 30-, 40- and 50-gallon units although you can find larger ones if necessary. You can find out how large your current tank is by looking at the nameplate. A good rule of thumb is that you can usually replace your old one with a new one of the same capacity unless you've recently added a bathroom, dishwasher or washer or are planning a new addition or appliances in the near future. You might also want to take advantage of the newer, more energy-efficient models.
Gas vs. electric: In general, gas water heaters win the battle. A water heater's capacity is a result of two factors: storage and recovery time (how quickly it can heat a tankful of water). With recovery, both gas and electric water heaters are rated by the number of gallons of water they can raise to 90 degrees F in one hour.
For example, a 40-gallon gas heater rated at 40,000 BTU can "recover" more than its entire capacity (41 gallons) in one hour. A similar capacity electric heater running at 240 volts will only heat half of its capacity (20 gallons) in an hour. Propane heaters are most often found in rural areas.