Informational Guide

Sizing a Tankless Water Heater

It’s important to have the right-size tankless water heater for your home. Read on to find out what size tankless water heater you need.

by Josh Mitchell

Tankless water heaters are an efficient way to heat the water in your house. The good thing about a tankless water heater is that it takes up less space and is more efficient at heating water.

But, you need to make sure that you have the right-size tankless water heater for your home. Read on to find out what size tankless water heater you need.  

There are two main types of tankless water heaters: gas and electric.  

  • Gas heaters
    Gas heaters are very popular because they can heat water efficiently and quickly. A gas heater uses less energy to heat water and takes up little space. Gas heaters can reduce your water usage, and there are low-emission gas heater models. Gas units have special installation requirements, though, such as a need to be vented. 
  • Electric heaters
    Electric tankless water heaters are generally less costly than gas heaters, and you do not need to vent them. The main downside is that electric water heaters can get expensive, especially during colder months. You also need to make sure that your home’s electrical grid can handle the amperage that the heater unit requires.

A gas heater is probably better overall, but electric heaters are more convenient at a certain scale.  

Types Of Tankless Water Heater

What Size Tankless Water Heater Do I Need? Sizing Guide from a Plumber 

There are several factors that determine what kind of water heating system you need. Here are a few of them.  

Size of Your Home 

Virtually all modern tankless water heater systems can heat a full-size single-family home entirely. Smaller models can be inefficient, and the water flow will be restricted to a relatively small location in your home. The benefit of tankless water heaters is that you only heat as much water as you need and no more. 

As such, you will end up saving on water costs than you would with a regular tank water heating system. The size of the heater you choose needs to not be so small that it cannot heat all the bathrooms in your home.  

Flow Rate in Your Home 

The flow rate is a measure of how much water your tank can move per minute. Flow rates are measured in gallons per minute (GPM). You need to match the maximum flow rate with the amount of water you want to use at once.  

Here is a quick measure of how much water common home fixtures use.  

  • Showers - 1.8 GPM 
  • Faucet - 1.0-1.5 GPM 
  • Dishwasher - 1.5 GPM 
  • Washing machine - 3-5 GPM 

The flow rate of your home directly determines what size tankless water heater you need. The larger your flow rate, the more powerful of a machine you will need.   

Required Temperature Rise 

You also need to determine how hot you want your tank to produce. The temperature rise is the difference between the temperature that the water enters your home and the temperature you need it to be. So if the water comes into your house at 50 degrees F and your shower needs 105 degrees F, the temperature rise is 55 degrees.

Given that a shower takes 1.8 GPM, your tankless heater needs to handle heating at least 1.8 gallons of water by 60 degrees every minute. The average ground temperature of water in the US is about 40 degrees F. 

Existing Plumbing in Your House 

A tankless water heater might require a new installation, so you need to consider how well it will fit with the existing plumbing in your house. A tankless water heater usually requires minimal space, so you should not have too many space requirements to install it. 

You can often install a tankless unit directly onto your existing gas or electric tank unit. If you use an electric tankless unit, then you need to consider your existing electrical setup in your home. For example, a gas-powered system may require the installation of an additional gas line in your basement or back room.  

What Size Tankless Water Heater Do I Need

What Size of Tankless Water Heater Do I Need for a Family 

Obviously, the size of your home is one of the primary things that determine the size of the water heater that you need. As a general rule of thumb, your heater needs to be able to heat up to 20-30 gallons per person living in the house.

So if there are 2 people living in the house, then it needs to heat at least 60 gallons. If you use a lot of water, then you need one that can handle about 40-50 gallons per person. The good thing about tankless water heaters is that they only heat as much water as you need. 

Replacing Storage Tanks: Tankless Water Heater Size Equivalents 

All other things being equal, you can convert between sizes of tank water heaters and tankless water heaters with the conversion of 1 GPM per 10 gallons of the tank. So, for example, if you have a 30-gallon tank, then you would need a 3 GPM tankless heater to get the same water capacity and flow.

Likewise, if you have a 50-gallon water tank, then you would need a 5 GPM tankless heater or higher. The larger the tank you have, the larger the tankless heater unit you should get. An 80+ gallon tank will need a 10-11 GPM tankless heater or larger to match the water and flow requirements. 

Tankless Water Heater Size Equivalents

Electrical Requirements for Tankless Water Heaters 

Generally speaking, an electric tankless water heater requires about 120 amps to run properly. That means that most people will have to substantially upgrade their electrical systems to work.  

You also need to make sure that you have the right breaker size and gauge wire for your system. For example, if you have a 7.7 kW heater, then you will need 40 amp breakers and 8 gauge wire.

For a heater greater than 11 kW, then you will need 60 amp breakers and 6 gauge wire. Heaters larger than 13 Kw require at least 2 60 amp breakers and 6 gauge and wires.  

Popular Tankless Water Heating Brands 

  • Navien Tankless Heater
    Navien tankless heaters are known for their Japanese quality engineering. All tankless models from Navien are gas-powered and can be modified to burn either natural gas or liquid propane. All models meet the Low-NOx standard where reduced emissions are required and have residential warranties. All Navien tankless heating units are Energy Star compliant as well. 
  • Rinnai Tankless Water Heater
    Rinnai tankless water heaters are known for their heating coil design that is highly efficient and reliable. Rinnai heaters save up to 40% on energy and have 0 CO2 emissions. They are one of the best water heating manufacturers in the country. They also have a professional service that monitors your usage and efficiency. 
  • Rheem Tankless Water Heater
    Rheem is known for its extremely durable and energy-efficient tankless water heating systems. They make an extensive lineup of water heating systems, so there are several versions to choose from that all have their pros and cons. Most of their heaters have Energy Star ratings, so they are highly efficient. 

People also Ask (FAQs)

Can you run out of hot water with a tankless water heater? 

Technically no, it will not run out of water, but you might be using water faster than it can heat up. Tankless water heaters take up water as you use it.  

What size tankless water heater do I need for a mobile home? 

You will probably need at least a 3 GPM tankless water heater for a mobile home. You may also have to install the proper gas hookup too.  

How long do these types of water heaters last? 

A good-quality tankless water heater can last over 20 years if taken care of properly and cleaned regularly. Tankless water heaters need to be flushed at least once every 12 to 18 months.  

Can I install a tankless water heater myself? 

You can, but it is not recommended unless you have extensive plumbing experience installing water systems. Professional installation is the recommended option.  

How much does it cost to install a tankless water heater via a plumber? 

It can cost between $600-$2,500 to install a tankless water heater unit. In addition, gas tankless heaters may require the installation of a new gas line, and electric heating units may require modifications to your home's power grid.  


Tankless water heaters are a great option to increase the efficiency of your water heating system. A good tankless heating system can save you money in the long term and reduce energy costs on heating water.  

Josh enjoys researching, testing and diving into home improvement & DIY products. He has a passion for tools, learning new skills and fixing the everyday problems that arise around the house.