What Size Tankless Water Heater Do I need -The Easiest Guidelines

Tankless water heaters are house appliances that heat water directly without using a storage tank. They provide hot water on demand hence less electricity consumption.

Although these heaters are easy to install, maintain and have a longer life span, they can only heat a limited amount of water at a time. This means you need to be extra careful when choosing one for your home – you need to get the right size.

So, why is it important that you get the appropriate size?

If you choose a small tankless water heater, it won’t supply you with enough water to heat your home. Get a bigger size, and you’ll have to bear with the extra costs.

If looking for a tankless water heater for your home but are confused about the size to get, this guide will help you a great deal.

Sizing Guide for a Tankless Water Heater

To figure out what size tankless water heater you require for your home, you need to consider the following factors.

What Size Tankless Water Heater Do I need

Size of Your Household

The size of your family will significantly determine the size of the tankless water heater to purchase for your home. The larger the size of your household, the larger the size you need to get and vice versa. Obviously, a family of 2 will require a smaller heater than that of 6.

Although your family size matters a lot, you also need to consider instances of high water usage. It doesn’t matter whether you have a small or large family; there times when you’ll experience a peak of heat water usage.

Think about your siblings’ birthday parties, do you usually invite friends over? What if you have a wedding and need people over to help you plan everything, you’ll have guests over from time to time, right?

The more the number of guests, the higher the rate of hot water usage. And although you might not be using hot water in your home at all times, you still have to think of those times when the usage skyrockets.

Estimating the tankless water heater size based on the highest possible usage will get you covered at all times.

Size of Your House

Since tankless water heaters come in different sizes, you need to think about the best size for your big or small house. You should know that opting for a small heater might not be a great choice when you have a big house.


You might be restricted to heating water that only flows to certain bathrooms in your home.

The best thing about getting a well-sized tankless water heater is that you get to store and heat as much water as you need. The size you select should not be too small to output less heated water than your need in every room.

Rate of Water Flow in Your Home

To get the right-sized tankless water heater, you need to figure out your maximum hot water needs. To do so, you need to be able to tell how much water per minute the heater can heat up.

The size of the tankless water heater you get determines its capacity, flow rate (measured in GPM), and its ability to meet your demand for hot water.

The good news is that most tankless water heaters come with predetermined flow rates. Small water heaters have low flow rates, while bigger ones feature a larger water flow rate.

To correctly determine the water flow of a tankless water heater to get for your home, you need to;

  • Get the number of showerheads, faucets, and appliances that you need to run through the tankless heater and their flow rate in gallons per minute.
  • Add up the above. The sum will give you the flow rate of the water heater to go for. You, however, need to keep in mind that you’ll rarely use the devices simultaneously. To avoid under-sizing your unit, assume that they all run together.

Take for example, if you have a showerhead that uses 1.6 GPM, faucets with flow rates of 1.4 GPM, and a laundry washer with a flow rate of 2.0 GPM, you need a heater with a maximum flow rate of 10 GPM.

Temperature Rise

Now that you’ve calculated the water flow, you need to determine the amount of heat your heater has to generate to give the required heat in the water outlets.

To do this, you’ll have to measure the temperature of the water coming through your faucets. All you require is a simple thermometer, and then subtract that from your desired output temperature.

For instance, if the incoming water temperature is 45 degrees F and you want an average shower temperature of 100 degrees F, go for a heater with a temperature rise of at least 55 degrees F.

How Efficient Are Tankless Water Heaters?

Tankless water heaters are among the few tools that create a more energy-efficient home. Unlike other units requiring you to heat and reheat water so that it remains hot at all times, these heaters instantly generate warm water.

Although the heating requires more power, you don’t have to reheat the water time and again. This translates to less energy use. According to Consumer Reports, tankless water heaters are 22% more efficient when compared to traditional units.

These heaters also have a longer life span when compared to standard water heaters and have lower month-to-month costs as well. They are much smaller hence occupy less space. Actually, installers tend to mount them on walls.

If you have a smaller home with minimal demand for hot water, a tankless water heater comes in handy. It’ll provide enough water for you to wash clothes, shower, and do dishes.


After going through this article, you should be able to purchase the right-sized tankless water heater for your home. From measuring the GPM rate to calculating temperature rise, the guide has it all. All you need to do now is ensure that you get the right size to cater to your household’s hot water needs. Best of luck!

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