Choose What Size Water Heater You Need Like a Pro


water heater size

When you’re on the hunt for a new water heater, one of the first things to think about is the capacity of the water heater (ie the number of gallons the tank holds). Conventional wisdom would tell you to get the largest heater possible. However, it depends more on the number of people in your household and the recovery ability of the water heater.

So, what size of water heater do l need? It’s generally recommended you have 10-15 gallons of hot water per person in your household. When shopping for a new water heater you should look at the capacity of the water heater, the water heater’s First Hour Rating (FHR), and your personal Peak Hour Demand calculation. A general estimated water heater size you will need is:

  • Families of 2 will need a 30-gallon water heater
  • Families of 2-3 will need a 30-40 gallon water heater
  • Families of 3-4 will need a 40-50 gallon water heater
  • Families of 4-5 will need a 50-60 gallon water heater
  • Families of 5 or more need a 60-80 gallon water heater

These numbers are only estimates and will vary greatly based on how much hot water you use within your busiest hour.

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The Peak Hour Demand calculation is the amount of hot water your home is likely to use within a busy one-hour period.

The First Hour Rating (FHR) is the amount of hot water that the heater can produce in one hour. You should also think about the fuel source and its physical size.

To learn more about choosing the correct water heater size, read on. 

Determining Tank Size Based on Family Size

water heater needs

Tank-style water heaters have a storage capacity of 30 to 80 gallons.

For most homeowners, 40-60 gallons is sufficient. But again, it depends on your overall hot water needs. 

Some households just use more hot water than others.

To get the right size for your household, you need to do the math. Here’s a rough estimate to get you started:

  • Households with one to two persons will likely need at least a 30-gallon heater capacity.
  • Two and three-person families need at least 40 gallons.
  • For a three and four-person family, a 40-50-gallon tank is sufficient. 50 gallons is good if you’re using electricity, and 40 gallons will work for natural gas or propane.
  • Families with more than five persons may benefit from an 80-gallon electric water heater or 50-gallon gas water heater.

Keep in mind the list above is just a guideline; hence, you cannot draw solid conclusions based on it. Hot water needs vary from one household to the next.

A three-person family, for example, might find a 40-gallon water heater isn’t enough to meet their needs, while the same tank capacity might do the job for a 5-person family.

This clearly indicates the family size isn’t the only factor you need to look at. 

Some people take longer showers than others, or some households have too many appliances and fixtures drawing hot water from the water heater. So, it’s a matter of how much water a household uses per day.

To calculate your typical usage, you can start with these pointers:

  • The number of people showering, and the time at which they are showering. 
  • If the major appliances are used simultaneously with people showering.
  • The fill capacity of the major appliances used.
  • How the bathtub is used. Do you fill the tub halfway or all the way?
  • Are you planning to remodel your bathroom or kitchen any time soon? If so, will you be getting a larger bathtub? 
  • Since a water heater will last up to 15 years, are you planning on having more children or getting married soon? This will change the demand for the hot water.
  • For large homes, consider installing one water heater for every 2 bathrooms or one water heater per floor.

Evaluating the Peak Hour Demand and First Hour Rating

Now that you’ve completed the lifestyle audit above, figuring out the Peak Hour Demand (PHD) and the First Hour Rating (FHR) will be easy. This information will help you get a water heater that keeps up with your hot water needs.

What is the Peak Hour Demand? The peak hour demand is the amount of hot water used during rush hour use. It’s basically determining the time of day that you use the most hot water at the same time in your home. 

Your peak hour may be 8pm at night before getting everyone ready for bed or at 6am when everyone is hurrying to get ready for school or work. Simply, its the hour of the day the most hot water is consumed.

What is the First Hour Rating? The First Hour Rating (FHR) is the amount of hot water that the heater can produce in one hour. Keep in mind this not the same thing as the amount of water the tank can hold. 

Also, gas water heaters typically have a higher FHR than electric units.

The FHR depends on the fuel source, tank size, and the size of the burner. You can find the FHR information on the Energy Guide Label of the water heater.

Below is a checklist to help you estimate your peak hour usage. Keep in mind the peak hour demand should be a little lower than your heater’s first-hour rating.

  • Hair shampooing (per household member): 4 gallons
  • Dishwashing by hand: 4 gallons
  • Face/hand washing (per household member): 4 gallons
  • Shaving: 2 gallons
  • Showering (per household member): 10-15 gallons
  • Automatic dishwasher: 14 gallons
  • Automatic washer: 10-30 gallons (Older clothes washers can use up to 45 gallons of water while modern energy efficient washers use as little as 5 gallons)

For a family of five, for example, here’s how you can calculate your peak hour demand. If every morning, three people shower, two face wash, and one shaves and washes the dishes by hand, 74 gallons is the average amount that will be used.

So, if your peak hour demand is 74 gallons, you should look for a water heater with an FHR of 76-80 gallons. 1

Sample Worksheet for Estimating Peak Hour Demand/First Hour Rating *

USE AVERAGE GALLONS OF HOT WATER PER USAGE   TIMES USED DURING 1 HOUR   GALLONS USED IN 1 HOUR
Shower 10 ×   =  
Shaving (.05 gallon per minute) 2 ×   =  
Hand dishwashing or food prep (2 gallons per minute) 4 ×   =  
Automatic dishwasher 14 ×   =  
Clothes washer 10 ×   =  
      Total Peak Hour Demand =  

My Families Personal Example

3 showers 10 x 3 = 30
2 shave 2 x 2 = 4
1 Automatic dishwashing 6 x 1 = 6
1 Clothes washing 10 x 1 = 10
Peak Hour Demand       = 50

*The above worksheet is based on my families personal usage. Your families peak hour usage will likely vary.

Sizing a Tankless Water Heater

Tankless water heaters aren’t as common as tank-style units. However, they are slowly gaining in popularity simply because they take up less space and can reduce energy costs by as much as 25%.

If you opt for a tankless water heater, however, a different set of factors come into play. These units don’t store water; hence, the tank’s capacity is the least of your worries. What you should pay close attention to, however, is the flow rate and temperature rise.

To find the perfect tankless water heater for your family, you need to estimate the water flow rate inside your home and the temperature change. Flow rate is usually measured in gallons per minute(GPM).

Use the following information to get an estimate of the flow rate in your household.

  • Dishwasher: 1.5 GPM
  • Washer: 2 GPM
  • Shower: 2.5 GPM
  • Kitchen/Bathroom Faucet: 1.5 GPM
  • Running bathtub: 4 GPM

You should implement the same calculations used in the estimation of peak hour demand. Add up all the flow rates of faucets and appliances used at the same time. 

If, for example, you’re running three showers and a dishwasher at the same time, you’ll need a tankless tank with a minimum flow rate of 9 gallons per minute.

The next step is determining the required water temperature rise. To do so, you need to subtract the incoming water temperature from the set output temperature of the unit, which is usually 100-120 degrees Fahrenheit.

Here are some incoming temperature estimations based on location:

  • 40 degrees Fahrenheit for the northern parts of the US
  • 50 degrees Fahrenheit for the southern parts of the US
  • 60 degrees Fahrenheit in the southwest, south California, and gulf states

If you still don’t know the incoming water temperature in your area, you can use the 50 degrees Fahrenheit estimation. 

If you want to be more accurate, you can turn on the cold water at your kitchen or bathroom faucet and let it run for a few minutes. Take a thermometer, stick the metal end into running water, and measure its temperature. This is a more accurate way of determining the incoming temperature.

Now let’s say the incoming/cold water temperature is 40 degrees Fahrenheit and the household hot water temperature is 120 degrees Fahrenheit. 

The required temperature rise would be 80 degrees Fahrenheit. This means you’ll need a water heater that produces a temperature rise of 80 degrees Fahrenheit at a flow rate of 9 gallons per minute. 

It’s a good idea to round up the numbers a bit to ensure your new water heater can meet your hot water demands. Fortunately, most manufacturers provide a flow chart on their sites.

Other Factors That Affect Water Heater Size

  • Inlet water temperatures vary by region and season. Summer doesn’t require as much input as in the winter. Hence you need to ensure there is sufficient hot water flow on the cold winter days.
  • The temperature rise of tankless gas water heaters is usually larger than that of electric units. Gas-fired tankless heaters can produce a temperature rise of 70 degrees Fahrenheit at a flow rate of 5 GPM, and the same temperature at 1.5 to 2 GPM through an electric model.
  • Some tankless water heaters are controlled by thermostats. These units provide flexibility because their output temperature varies according to the incoming water temperature and flow rate.

Final Thoughts on Water Heater Sizing

If you are still unsure of what size water heater you need for your family, consult with a licensed plumber for help. Another option is to look at the on-demand tankless water heater options. Check out our Buyers Guide: What Tankless Water Heater Do I Need.

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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