How to Choose the Right Water Heater Size

Tips to help you choose the correct water heater size.

Do you constantly run out of water or does your water heater break down often? Are you in need of an upgrade? Purchasing a new water heater is a little different from running to the store to pick up a few supplies.

It requires some thought and, in some cases, may feel a little daunting. We all want to get it right the first time and resume normal living, right? The question that’s likely at the forefront of your mind is, “What size do I need?”

In this article, we’ll be sharing some things to consider when finding the right size water heater for you. We’ll also show you how to calculate your water usage and how that impacts on your water heater choice.

Types of Water Heaters

There are different types of water heater systems that you can choose from including:

1. Solar Water Heater Systems

Solar Water Heater Systems

These systems rely on the sun’s renewable energy and incur low operational costs compared to the standard gas or electric water heaters. However, many homeowners shy away from them due to their high upfront installation costs.

The main components of this system are the storage tank and solar collector. The solar collector is the flat box made of a transparent cover and an insulated backplate. Within the collector, you’ll find several tubes that carry a coolant (1).

For your heater to perform optimally, you need to consider how much of the sun’s energy reaches your building. The placement and tilt of the collector determines how efficient your heating system will be.

The best thing about these water heating systems is that they use both direct solar radiation and diffuse solar radiation. Those of us who don’t live in sunny climates can still have access to an adequate solar resource.

This is especially true if the building is un-shaded and generally faces south. Before installing the solar system, it’s important to have the supplier or installer come over.

They’ll evaluate your building’s access to the sun’s energy and advise you on the size and type of system needed. You’ll need one that can meet over 90 percent of your household’s hot water needs in the summer.

On cloudy days or whenever your hot water usage increases, you’ll need a backup system. This can be a traditional water tank or a tankless system.

Solar hot water systems are low maintenance and may need a checkup every three to five years. It’s likely that you may only need to replace a part or two after 10 years.

Optimal placement is a key consideration of the solar system’s collector. Think about the following before purchasing a solar hot water system:

Size of Solar Collector

In most cases, the installer or contractor will follow a guideline of roughly 20 square feet for the first two people. For every extra person, they add eight square feet if you live within the U.S. Sun Belt region.

For those living in the Northern United States, the contractor will add 12-14 square feet for every extra person.

Storage Volume

Here are some estimates that you can work with:

  • A 50-60 gallon tank can serve a household consisting of one to three people.
  • An 80-gallon storage tank can work well for a family of three to four people.
  • Tanks exceeding 80 gallons are ideal for four to six people (2).

Building Codes and Regulations

Before installing a solar water heater, it’s important to liaise with the municipal office. Also, talk to your neighborhood, community and homeowners association (whichever applies) and let them know your intentions.

They’ll provide guidelines and restrictions you’ll need to adhere to. Examples may include:

  • Exceeding roof load.
  • Erection of unlawful protrusions on roofs.
  • Improper wiring.
  • Obstruction of side yards.
  • Siting your system too close to lot boundaries or streets.

Always remember to make any copies of approvals that you receive for future reference (3).

2. Tankless Hot Water Systems

Tankless Hot Water Systems

These hot water systems are gaining popularity in the US due to their efficiency and decreased energy cost. They only generate hot water when needed. Tankless systems use either an electric element or a gas burner to heat the water.

When you open the hot water faucet, cold water moves through the pipe and into the unit. It’s heated, then flows out as hot water.

A key advantage of this system is that hot water is instantaneous — there’s no waiting for a storage tank to fill up before use. Most tankless water heater systems are hung on walls, meaning they can easily fit in tight spaces.

Unlike tank-types, there’s no risk of springing a leak which can lead to a loss of gallons of water. You also won’t have to worry about the tank tipping over in an earthquake if that’s a possibility. More importantly, the Legionella bacteria won’t make their home in your system (4).

But they aren’t without downsides. The output in a tankless water heater limits the flow rate. Generally, these heaters provide 2-5 gallons of hot water per minute which may not be enough for simultaneous and multiple uses.

What this means is that you may not be able to run the dishwasher and take a hot shower at the same time. For you to do so will require the installation of two or more water heaters.

Factors That Affect Sizing

Flow rate, as well as temperature rise, are the two main considerations when sizing a tankless water heater.

Flow Rate

To determine the desired flow rate from your heater, calculate your peak hot water usage. This is the number of hot water appliances you expect to use simultaneously.

Think dishwasher, shower and washing machines. Now add up their flow rates which are essentially how many gallons they use per minute(gpm). Here’s an example to help put things in perspective:

  • Shower – 2.5 gpm.
  • Sink faucet – 1 gpm.
  • Bathtub – 3 gpm.
  • Dishwasher – 3 gpm.
  • Washing Machine – 3 gmp.

Now let’s assume your peak hour is in the morning. Someone will be in the shower while you’re running the dishwasher and one other person will be shaving. Your usage is 6.5 gpm which means you need a heater with a minimum of 6.5 gpm flow rate.

Temperature rise

An easy way to do this is to let the cold water faucet run for a couple of minutes then use a thermometer to gauge its temperature.

On average the hot water temperature for households is around 110 degrees Fahrenheit. Now, subtract the cold water temperature from the 110 degrees Fahrenheit to arrive at the required temperature rise.

If, say, your cold water temperature is 65 degrees Fahrenheit, subtract this from 110 degrees Fahrenheit. 45 degrees Fahrenheit, therefore, is how much the heater needs to heat the water in order for it to arrive at 110 degrees Fahrenheit.

In other words, the temperature rise required is 45 degrees Fahrenheit (5).

All tankless water heaters have a chart which stipulates hot water flow at various temperature rises. Use the info to adjust your heater accordingly.

3. Tank-Type Hot Water System

Tank-Type Hot Water System

These conventional storage tanks are popular in many homes across the U.S. and offer a ready reservoir of hot water. Whenever a person opens a faucet, hot water flows from the top of this tank. At the same time, cold water enters the tank’s bottom.

This ensures that the tank remains full. Unfortunately, since the water is constantly being heated, whether you’re using hot water or not, energy is lost. This is referred to as standby heat loss.

So, what size tank water heater do I need? There are several factors to consider when choosing the right size for your needs. They include:

Physical Size

If you’re replacing your current tank with another of similar size then this isn’t an issue. However, circumstances change and you may find that your growing family uses more hot water in the future.

If you need to purchase a larger tank, think about the space you’ll need to fit it in.

Fuel Source

Conventional tank-type water heaters use different fuel sources, including liquid propane, natural gas, fuel oil, and electricity.

The fuel type and its availability affects the size of the water heater you want to purchase. It’ll also affect its energy efficiency and annual operational costs (6).

Household Size

A large household is likely to use more water than a home of two. However, it’s worth noting that the size actually depends on how much water your property uses — you can have a small family that still consumes more water than a larger one. Some general guidelines for tank capacity are as follows:

  • 30-40 gallons for one to two people.
  • 40-50 gallons for two to three people.
  • 50-60 gallons for three to four people.
  • 60-80 gallons for five or more people.

These figures are just estimates and if you want a more accurate picture, you’ll need to do some math. Here’s what you’ll be looking at:

Peak Hour Demand

Think about what time of day you use hot water the most in your home. Also, bear in mind the number of family members living in your home.

First Hour Rating (FHR)

This refers to how much hot water the water heater can produce in a single hour. This figure indicates the heater’s efficiency and how much it can handle at peak hour.

You can use the worksheet below to come up with a better estimate of your peak hour demand. This will help you know your FHR.

Use Average Gallons of Hot Water Per Usage Times used In an Hour Gallons Used Per Hour
Shower 10 X X
Shaving (.05 gpm) 2 X X
Hand dishwashing or food preparation (2 gpm) 4 X X
Automatic dishwasher 6 X X
Washing Machine 7 X X
total Peak Hour Demand


4 Showers 10 X 4 = 40
1 Shave 2 X 1 = 2
1 Auto dishwashing 6 X 1 = 6
Peak Hour Demand 48

The above example shows a peak hour demand of 48 gallons. This means that this family needs a water heater with an FHR of 50 gallons.

You’ll want to choose a model with an FHR that meets or exceeds your water usage at the busiest time of day (7).

On a lighter note, there are some people who don’t believe that a shave can consume two gallons of water. Well, those who live in larger homes may attest to this, but we’ll leave that up to you!

Sizing Up Your Options

What size water heater do I need? We believe you’re now better placed to answer this question. Know your household’s hot water usage — then you’re well on the way to making the best choice.

Do you have any questions or comments? We would love to hear from you, and remember to hit the share button.

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About the Author

Peter Gray

Peter has been a homeowner for 35+ years and has always done his own repair and improvement tasks. As a retired plumber, Peter now spends his time teaching others how they can fix leaks, replace faucets, and make home improvements on a budget.