What Size Water Heater Do I Need?

If you are considering replacing your water heater, the pressing question may be, “What size water heater do I need?”

With residential water heaters ranging from 40 to 100 gallons, there are a few factors to consider before purchasing a new one.

Household sizeWhat Size Water Heater Do I Need?

If you have a large household and are often running simultaneous or back-to-back showers, loads of laundry or dishes, you will want to invest in a larger capacity hot water heater. However, if you have just a few people living in the home, purchasing a water heater with more capacity than is needed will just waste energy. Here are some other helpful facts to take into consideration.

Tank’s FHR (First Hour Rating)

Hot water heaters are rated with an FHR to tell the consumer how much hot water the tank will supply in the first hour of use, when the tank is fully heated. The Federal Trade Commission requires the FHR be listed on the yellow Energy Star label on new appliances. The higher the number on the FHR, the quicker the hot water heater will replenish the used hot water.

Some of the factors that affect FHR are the size of the hot water tank, the fuel source (which is usually electricity or gas) and the size of the hot water heater’s element.

Average Water Usage

To break down hot water usage into mathematical terms to make some quick calculations, here are some water usage averages according to U.S. Geological Survey.


  • Using a water-saving shower head = 2 gallons per minute x 10 minutes = 20 gallons
  • Using a non-restricted shower head = 5 gallons per minute x 10 minutes = 50 gallons


  • A full tub averages 36 gallons of water

Tooth brushing:

  • Newer faucets = 1 gallon per minute
  • Older faucets = 2 gallons per minute


  • Newer dishwashers with Energy Star ratings = as low as 6 gallons per wash cycle
  • Older dishwashers = up to 16 gallons per wash cycle

Clothes Washer:

  • New clothes washers with high Energy Star ratings = 25 gallons per load
  • Older clothes washers = up to 40 gallons per load

Peak Demand

Adding up all of the gallons of your household’s peak water usage will assist in determining an appropriate size water heater tank for your household needs. For example, two water-saving shower head ten-minute showers would total 40 gallons. If your household has more than two people showering in the morning, it may be necessary to purchase a larger water heater than the minimum 40 gallons.

If you have multiple people showering and typically start a load in the dishwasher, this will potentially be 40-60 gallons, plus another 6-16 gallons dependent on the energy and water efficiency of your dishwasher. Even with the water heater working to recover the used hot water, there could easily be an overage in the amount of hot water available leaving someone with a cold shower. Underestimating the hot water heater size needed will certainly leave you frustrated.

Once you total the peak demand for your household, it is recommended to find a water heater with a capacity within one to three gallons of the total. The bigger the water heater, the more energy will be used to heat the water daily so try not to overestimate.

Energy Star Rebates

For more information on water heater Energy Star Products and potential rebates in your state, visit Energy Star.

Contact Us

Contact Pilchuck Heating for more information on energy savings and for all of your heating and cooling needs.


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