What Size Tankless Water Heater Do I Need?

Getting the right size for your tankless water heater is important. Too big, and you end up paying extra up front unnecessarily. Too small, and you risk not having enough hot water to go around.

How Are Tankless Heaters Sized?

Unlike traditional tank-based water heaters, tankless units are not sized by how much water they can store. Instead, they are ranked based on flow rate. This is the measure of how much heated water can be output from the heater as measured in GPM (gallons per minute).

You can actually calculate how much GPM you will need based on your household size and amenities usage. There are multiple online tools to help you estimate this or use the table below. The more water you are likely to be using at any given time translates to you needing a larger GPM and model.

To use this table to calculate your needed GPM, add up all the flow rates for the heaviest water use you expect in a single moment. If you think there will be 2 standard shower heads, 2 faucets, and a high efficiency dishwasher in use at the same time, the GPM would be 7.5 (2+2+1+1+1.5).

Type of FixtureFlow Rate
Standard Shower Head2 GPM
“Rain” Type Shower HeadUp to 5 GPM
Body Sprayer (per nozzle)2 GPM
High Efficiency Dishwasher0.5 – 1.5 GPM
Standard Dishwasher 2.5 GPM
Faucet1 GPM
High Efficiency Washing Machine1 GPM
Standard Washing Machine2.5 GPM

How Powerful Does My Water Heater Need To Be?

In addition to being honest with yourself about how inefficient appliances may be or how hot you like your showers, there are also environmental factors to consider. If you live in a region with colder temperatures, then the water the heater intakes will need more energy in order to be warmed. This temperature rise, also known as ΔT (pronounced ‘delta tee’), is inversely related to the ability of your heater to supply your needs.

This means that people in a colder climate who need to heat the water more will be able to supply fewer appliances than if they were to live in a warmer climate. Hawaii residents are fortunate for having to worry less about this than many other Americans.

Finding the right size of tankless water heater for you and your family is easier with some expert assistance. That’s why EPS is here! Call us at (808) 691-9309 Oahu, (808) 400-8811 Hawaii Island.


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