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 higher hot water use than average.

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. Again, the amount of hot water depends on the temperature you choose.

Water 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.

-- Tips courtesy of HomeAdvisor.com