What Size Water Heater Do I Need? The Ultimate Guide for Determining What Size Water Heater You Need for Your Home
If you’re looking for a new water heater for your home, choosing the right size can make a world of difference when it comes to your utility bills and comfort. Pick one that’s too big, and you’ll be paying to heat water you’re not using. Go too small, and you risk running out of hot water when you need it.
So, how do you make sure that you’re getting the right water heater that will suit your needs and not drive up your energy bills?
Unfortunately, this involves some math – but it’s basic. Your options will also vary depending on the type of water heater you plan to purchase: tank or tankless. The needs for these are measured differently.
Correctly Sizing a Tank Water Heater
A storage tank water heater is still the most common type of product that stores your hot water until you need it. These have lower upfront costs, but may not be as energy efficient as a tankless water heater. Because these are scalable, they are suitable for larger households but can be limiting if you choose one that is too small.
As a general rule, the more hot water your family uses at once, the larger capacity tank you’ll need. Let’s assume that you want to skip the math and just get a rough estimate.
For households between 1 and 2 people, you’ll need at least a 30-gallon capacity tank.
For households between 2 and 3 people, you’ll need at least a 40-gallon capacity tank.
For households between 3 and 4 people, you’ll need at least a 50-gallon capacity tank.
For households with 5 more people, you’ll need at least a 60-gallon capacity tank.
The above are general guidelines that might not accurately match your family’s hot water needs. For example, a family of 4 might be able to comfortably use a 40-gallon tank, and a family of 2 might find that a 30-gallon tank isn’t nearly enough. This is why it makes sense to look at hot water consumption.
To get the most accurate picture of your household’s hot water consumption, you’ll need to follow a few simple steps:
Determine your “peak hour demand.” This is the one hour during a typical day in which you use the most volume of hot water. If everyone is showering, shaving, and doing dishes between 7:30 a.m. – 8:30 a.m. in the morning, this is your peak hour.
Calculate your hot water usage during your peak hour. The S. Department of Energy provides a table/worksheet that allows consumers to estimate their peak hour demand in gallons of water.
Average Gallons of Hot Water Per Usage
Times Used During One Hour
Gallons Used in One Hour
Shaving (.05 gallon per minute)
Hand dishwashing or food prep (2 gallons per minute)
Total Peak Hour Demand
1 hand dishwashing
Peak Hour Demand
In the example above, the peak hour demand is 46, so it is best to choose a tank water heater with at least a 50-gallon capacity.
Choose a tank with an FHR that matches your need. The FHR of a hot water heater is its “first hour” This is how many gallons of hot water the tank can heat in any given hour. When shopping for a water heater, the FHR is listed on the EnergyGuide label as well as other product specs.
What Size Tankless Water Heater is Best?
Tankless water heaters work differently their tank counterparts, and they are sized differently as well. They only heat water on demand, but how fast they will be able to keep up with your needs is what matters the most.
When you measure tankless water heaters, you’ll need to figure out two things:
Flow rate. This is how many gallons of hot water you might use simultaneously.
Temperature rise. This is how many degrees the incoming water will need to be heated.
Some typical household flow rates are:
Appliance Using Hot Water
Flow Rate (gallons per minute)
1.0 – 2.0 GPM
0.5 – 1.5 GPM
Here are the steps for figuring out the right size tankless water heater for your home:
Choose the hot water devices that you might use at the same time.
Add the average flow rates for those devices to get your home’s flow rate at peak time.
Based on your location, determine how much the water will need to be heated. For example, the average groundwater temperature in Southern California is around 65 degrees Fahrenheit.
For example, say you expect to run two showers, the kitchen faucet, and a bathroom faucet at the same time. Your peak household flow rate would be 7.0 GPM. Now assume that the hottest water you’ll need is 105 degrees (average) for the showers. The temperature rise is (105-65 = 45).
This means that you would need a tankless water heater with a minimum 45-degree temperature rise and a flow rate of 7 GPM.
Need Help with Your Next Water Heater Purchase?
This guide should help you determine what size water heater you need for your home, but this isn’t something you need to work through on your own. If you’re ready to replace your home’s water heating system and would like some assistance, Faster Water Heater Company can help.
We have served clients throughout the West Coast region for more than three decades and are available for water heater replacements 7 days a week (in most locations). Contact us now with any questions or to get started.