After heating and air conditioning, water heating is most homeowners' biggest energy expense. Getting the right-size water heater is crucial. Undersize the unit, and you won't have enough hot water; oversize it and you'll overpay to both purchase and operate the heater.

Use our calculators to estimate what size water heater you should buy. But remember to base your calculations on your typical period peak use—not on extraordinary days when you have a houseful of guests. You need to determine:

You can use our default values, which are based on American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and Consumer Reports' own research and testing. To get an estimate based on your particular water-use habits, calculate the gpm of your fixtures and appliances and input it into the calculator.

Note: Experts stress that a detailed consultation with an experienced plumber about your water-use habits is the best way to make the most accurate estimation of your heater size. These calculations are intended to provide a basis for such consultation by illustrating how particular incidents of water usage affect your total needs.