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Just Plumbing | Understanding Water Heater Size: Finding the Right Capacity for Your Household

Understanding Water Heater Size: Finding the Right Capacity for Your Household

Hot water is an essential in the home. We need it for laundry, cooking, bathing, and cleaning. This makes the water heater one of the most important appliances in the home. It’s even more important in regions where the weather is very cold. This appliance works by circulating cold water through heating coils in a tank.

A basic knowledge of the water heater system and how it operates isn’t enough. As a homeowner, you need to know the right size or capacity for your home. It will be a waste of money to purchase a water heater that’s under or far above the capacity of your home. While the former translates to hot water shortage, the latter means increased energy bills. Have you now seen why it’s essential to understand water heater size?

Well, of course, this understanding has to come from knowing the factors that determine the right capacity for your home, among other things. Let’s begin!

Key Factors That Determine Ideal Water Heater Capacity

If you want to make an informed decision on factors that will determine the right water heater size, look out for the following:

Number of Occupants

The number of people living in the home is the primary factor determining hot water demand. The more people in the house, the higher the water demand will be. Ultimately, this will lead to high water usage. General guidelines suggest sizing tank heaters at 20 gallons per person. So, a family of 4 would need an 80-gallon tank.

Just Plumbing | Understanding Water Heater Size: Finding the Right Capacity for Your Household


Number of Fixtures

Consider all fixtures and appliances supplying hot water. This will include sinks, showers, tubs, dishwashers, washing machines, etc. The more hot water outlets there are, the higher the potential demand. A house with four bathrooms will need a larger water heater than a house with two bathrooms.

Peak Usage Times

There’s no consistent water usage throughout the day. There are certain periods when we use water more in the home, which are mornings and evenings. That’s when we want to shower, wash plates, wash clothes, and cook. With this in mind, your size should not be according to the average daily need but rather the high demand during peak periods.

Flow Rates

Flow rate refers to gallons per minute for each water outlet. This includes showerheads, faucets, and so on. So, the flow rate for the entire home will be the maximum GPM in supply during peak usage periods. This will help to determine the right water heater size.

Climate and Water Temperature

Incoming groundwater temperature affects how much heat is needed to raise it to the desired level. Colder regions require more energy to heat water. Warmer climates may get away with smaller heaters. In summary, consider your location. A homeowner in Texas will need a smaller capacity of heater compared to a resident of Alaska.

Calculating and Estimating Water Usage in the Home

Even if you know the factors mentioned above, you still need to know how to calculate water usage in your home. This will help you to be more precise in choosing the right water heater size.

Imagine a family of 4 in the Midwest with two bathrooms. For a family of 4, a good rule of thumb is to estimate 60-80 gallons of hot water demand per day per person. With four people, that’s 240-320 gallons per day.

Now, let’s look at the morning peak demand. The master shower may use 2.5 gallons per minute (GPM). If it’s used for 10 minutes, that’s 25 gallons. The other shower uses 2 GPM for 8 minutes, which is equivalent to 16 gallons. Running the taps in the two sinks uses about 10 gallons. That sums up to 51 gallons just in the morning!

Add in other daily uses like dishwashers, washing machines, and sinks; the estimated daily demand fits in the 240-320 gallon range.

For a storage tank, we’d recommend at least a 50-gallon gas or electric model. A tankless should be sized at around 4-5 GPM, which can handle using multiple fixtures simultaneously. Whatever you do, always size up for a cushion because an undersized heater will lead to frustrations.

Types of Water Heaters and Typical Capacities

Storage Tank Water Heaters

  • Gas Storage Tank: Typical sizes range from 30 to 75 gallons. A 40 or 50-gallon unit is usually sufficient for a family.
  • Electric Storage Tank: Electric heating is less efficient than gas, so you’ll need 80-120 gallon tanks to meet high demand.
  • Heat Pump Water Heater: Most effective in warm climates. Around 50-80 gallons should do the job.

Tankless Water Heaters

  • Gas Tankless: You need 5-6 GPM to supply a whole house.
  • Electric Tankless: This is ideal for point-of-use applications and requires a lot of electricity.
  • Whole House Electric: These are larger electric tankless units (up to 12 GW) that can serve an entire home when properly sized.

Other Types of Water Heaters

  • Hybrid Heat Pump: This system combines a tank and heat pump for efficiency. Its capacities range from 50 to 80 gallons.
  • Solar Water Heaters: Solar-heated coils reduce energy usage and are ideal in places with erratic power supply. Tank sizes are like gas or electric heaters.

Installation Considerations for Capacity

Every aspect of your water heater system involves some key considerations. Even the installation process is not left out. Let’s look at what you should consider before installing the heater:

  • Available Space: The physical size of the water heater must fit in the installation location. So, if you’re installing a large-capacity tank, you’ll need more space. Closets may only accommodate a 40-50 gallon unit.
  • Gas Line Size: Gas tankless heaters and storage tanks require adequate gas line diameter and fuel input ratings to operate properly. Under-sized lines will impair the heating capacity.
  • Electrical Panel Amps: If you want to use larger electric heaters like whole-house tankless models, you’ll need very high amp loads. Sometimes, this may necessitate an electrical service upgrade.
  • Venting Needs: Gas water heaters require proper venting to the outside. The bigger the unit, the larger the vents and intake pipes you’ll need for the installation process.
  • Plumbing: The inlet and outlet plumbing must accommodate the required water flow rates.
  • Condensation Control: High-efficiency tankless heaters produce significant condensate that may require a drain or pump system.
  • Heat Load: In hot climates, the heat emitted from large tankless heaters requires outbound airflow.


Properly sizing a water heater ensures you meet your hot water needs. To recap, the key factors are the number of people, fixtures, peak usage times, and flow rates. We recommend calculating your optimal capacity using hot water demand estimates. As we mentioned earlier, oversized water heaters waste energy, while undersized systems cause shortages. Additionally, consider installation space, fuel/electric service, and plumbing limitations. Take the time to select the right water heater capacity for reliable flow and comfort.