Home » What Size Water Heater Do I Need?

What Size Water Heater Do I Need?

Have you ever asked yourself “what size water heater do I need”? If you are considering traditional tank style hot water heater, a standard 40-gallon electric water heater can supply enough hot water to meet the needs of most families. However, if your family uses more hot water than that — or you like to take long, luxurious baths — you may want to install a tankless water heater.

If your household is large or includes frequent users of dishwashers, clothes washers, and showers, you’ll want a 50-gallon or more model.

Remember to take into account the climate where you live, as well. If you live in a cold climate, you may need a water heater with higher insulation values to prevent heat loss. Always choose a water heater with a high enough PHD or peak hour demand rating to avoid needing another more large H2O tank.

Tankless Water Heater Sizes vs Traditional Tank Sizes

Tankless water heaters deliver 2 GPM all the way up to 12 GPM of hot water. How many GPM will you need? The 5-10 GPM ones work just fine for the majority of households. The water heater model you choose will vary based on the temperature rise, peak hour demand and first hour rating.

Tankless Hot Water Heater Size Graph

# Of Family Members Gas Tankless H2O Size (GPM) Electric Tankless H2O Heater Size (kW)
Family of 2 6 – 8 GPM 10-18 kW
Family of 3 7 – 9 GPM 15-23 kW
Family of 4 8 – 10 GPM 20-28 kW
Family of 5 9 – 11 GPM 25-34 kW
Family of 6 11+ GPM 34+ kW

Traditional water tanks are larger because they store the amount of hot water you could need at any given time. A family of 4 people should purchase a 50 or 60 gallon water heater. While a household with 6 or 7 people may want to consider a 70 – 80 gallon water tank.

Standard Gas Water Heater Size Graph

Traditional Water Heater Size By Family Size
Family Size Demand Gallon Capacity Required
Electric Gas
High 75
Regular/Low 80 50
3 – 4
High 80 50 – 75
Regular/Low 50 40
2 – 3
High 50 40 – 50
Regular/Low 40 40
1 – 2
High 40 – 50 40 – 50
Regular/Low 30 30

What Does Gallons Per Minute Mean?

Gallons per minute is a unit of flow in the US Customary system. There are 231 cubic inches in one gallon, so there are 7.4 gallons in one cubic foot. Therefore, 1 gallon per minute is equivalent to 0.00222801 cubic feet per second (cfs). Gallons per minute is an important unit of measurement because it will help you determine what size or type of water heater is the best fit for your home or commercial space. Gallons per minute is part of the formula used to determine your first hour rating and is relevant whether you decide to purchase a traditional water heater tank and tankless water heater.

Gallons Per Person Estimation Chart
People Gallons Low Range Gallons High Range
1-2 23gal 36gal
2-4 36gal 46gal
3-5 46gal 56gal
5+ 56gal 66gal + 10gal per person

How to Determine How Much Water Your House Uses

Water is a precious resource in California, and similar to temperature rise, it’s important to know how much water your household is using. Before you attempt to install a traditional tank or any size tankless water heater, use these tips on how to determine how much water your house is using:

  1. Check your water bill. Your water bill will tell you how many gallons of water your household used in the most recent billing cycle.
  2. Check your water meter. You can check your water meter to see how many gallons of water your household has used in the last 24 hours. Note: you will need to know how to read your water meter in order to do this.
  3. Check your appliances. If you’re not sure how much water certain appliances use, you can find out by checking the manufacturer’s website or looking for the Energy Guide label. The Energy Guide label will tell you how much water each appliance uses annually.
  4. Compare your usage to other households. It’s important to remember that not all households use the same amount of water. You can compare your water usage to others by looking at estimates from the American Water Works Association.
  5. Calculate your water usage. Check out this easy to use calculator developed by the United States Geological Survey. The USGS Calculator allows you to calculate how much water your household uses.
First Hour Rating Per Acttivity & GPM
Activity/Fixture FHR GPM Unit/Per Person
Bathing/showering 20gal 1.5 – 3gal 1
Washing hair 6gal .5 -.75gal 1
Washing hands 2gal .25gal – .5gal 1
Dishwashing by hand 6gal – 8gal .25gal – .5gal 1
Shaving 1.5gal – 3.5gal .25gal – .5gal 1
Running Dishwasher 12.5gal – 16.5gal .75gal – 1.1gal 1
Running Clothes Washer 27.75gal – 30.75gal .75gal – 1gal 1

Additional Factors to Consider While Sizing Water Heaters

Aside from peak hour demand, here are a few other factors that you need to keep in mind when sizing the water heater which we will discuss in this article.

Peak Hour Demand by First Hour Rating
Hand dishwashing/food prep (2gal per Min.) 3 × 2 6
Shaving (.05Gal per Min.) 3 × 2 6
Shower 20 × 2 40
Clothes washer (1gal per min) 16.5 × 1 16.5
H-Axis 15
1 15
Top-loader 25 1 25
Automatic dishwasher (1gal per Min. 7 1 7
Est. Peak Hour Demand 115.5

Surge Supply Installed Capacity

The size of the surge tank determines how much hot water can be supplied at any given time. If you do not install a surge tank, there is a good chance that the water heater will not be able to keep up with the demand, resulting in cold water.

Installation Location

The installation location of the water heater is also important to consider. If you are installing it in a tight space, you will need a smaller model. Likewise, if you are installing it in an area with a lot of sunlight, you will need a model that can handle the extra heat.

Water Temperature

The temperature of the water also needs to be taken into account when sizing the water heater. If you have a tankless water heater, it is important to make sure that the incoming water temperature is not too low, or the heater will not be able to generate enough heat to produce hot water.

By taking all of these factors into account, you can ensure that you are selecting the right size water heater for your home.

If you live in Sacramento area and are looking for more information on sizing water heaters, schedule a free consultation to find out what size water heater is right for you home, and water saving upgrade options.

Sacramento Plumbing Solutions will help you determine the following and more:

  • Hot water demand
  • First hour rating/gallons of hot water
  • Available water heater models
  • Storage tank size needs
  • Best tankless water heaters
  • Best storage tank water heaters
  • Tank Capacity
  • Heat pump options
  • Flow rate
Mike Grijalva

Mike Grijalva

Owner & Lead Plumber

I’ve been a journeyman plumber for over 15 years and opened my own plumbing company in 2011. I’m highly qualified and offer the best prices in the Sacramento area. I’ve done plumbing in every aspect possible, inlcluding new construction, service and repair, commercial, sprinkler service, and custom homes. I take great pride in my work.  You can feel comfortable trusting someone highly skilled with integrity for all your plumbing needs.

CSLB License #963048

Latest Plumbing Tips

Are Low-Flow Toilets Worth It?

Are Low-Flow Toilets Worth It?

I want to talk about a question I constantly hear from my clients. "Are low-flow toilets worth it?" I've seen a lot of changes in how we think about water usage over the years. But in my opinion, low-flow toilets are still one of the biggest game-changers when it...

Here’s Why Your Toilet Runs Randomly and What to Do to Fix It

Here’s Why Your Toilet Runs Randomly and What to Do to Fix It

As a journeyman plumber and the owner of Sacramento Plumbing Solutions, I've seen my fair share of plumbing issues. One of the most common problems homeowners face is a toilet that runs randomly. It's annoying and can increase your water bill significantly, and it's a...

The Disadvantages of Tankless Water Heaters

The Disadvantages of Tankless Water Heaters

Over the years, I've gained a ton of experience with different types of water heaters. One of the best types available today is tankless water heaters. A tankless water heater, or on-demand water heater, heats water without using a storage tank. When a hot water tap...