What Size Tankless Water Heater Do I Need? (A Comprehensive Guide)

Kenneth Wilson

Determining the best size tankless water heater is dependent on your total peak hot water demand and the coldest temperature of the supply water in your area. How many people live in your household, and which amenities and appliances will draw hot water at any point in time.

The table below depicts the minimum recommended size based on the number of people and the location of the household:

Mode of Heating

Gas (GPM)

Gas (GPM)

Electric  (kW)

Electric  (kW)

 Family Size

Southern US

Northern US

Southern US

Northern US


























Tankless water heaters are sized to deliver a certain volume of hot water per minute and are measured in Gallons Per Minute (GPM) for gas-heated systems and in kW for electrically heated systems. Let's look at what factors to consider when sizing a tankless water heater for your house.

Considerations For Ideal Sizing of Tankless Water Heaters

The investment in a tankless water heater will amount to $100 per 1 gallon per minute delivery of 110 degrees Fahrenheit hot water. The temperature of the supply of water is the first important factor to consider. States on the northern border of the US have a cold water supply temperature of 35 degrees Fahrenheit and will thus need to be heated by 75 degrees Fahrenheit to reach a delivery temperature of 110 degrees Fahrenheit.

The water temperature in the southern states of the US has a cold water supply temperature of 75 degrees Fahrenheit. It will only be required to be heated another 35 degrees Fahrenheit to reach the ideal delivery temperature.

The second factor to consider is how many hot water delivery points are designed into the house and how many people will live there. In a southern state, a household of four people, the recommended gas heated tankless water heater is rated at a size of eight GPM.

Typical Gallons Per Minute (GPM) required rates for southern and northern states:


GPM: Southern US 

GPM: Northern US










Washing Machine



In a four-person household, two separate showers can be running simultaneously as a dishwasher and washing machine for a total draw of 7.5 GPM. The same house located in a northern state would require an 11 GPM unit to supply the 10.5 GPM needed from the tankless water heater.

Based on the supply water temperature in the location of your house and the number of people in your household, you will be able to calculate the optimal size tankless water heater to buy. Install a water heater with some spare capacity to cope with peak hot water demand periods in the household.

Please don't run the dishwasher and washing machine when people take their showers or run a bath. Let others in the household know members when you run high volumes of hot water to avoid drawing more than the system's capacity. Draw up some rules for using hot and cold water more sparingly as a household to generate significant savings on water consumption.

The savings in water consumption in a household alert to the amount of water used per day can be far more significant than the energy savings of using a tankless water heater. As water scarcity has become more widespread due to climate change, the cost of water is rapidly rising.

The long-term drought in the Pacific Mid-West has crippled economic growth and is of grave concern. Even the northern states have been affected by the quality of drinking water supplies, as evidenced by Flint, Michigan.

Suppose you are concerned about the energy demand of your household for economic and environmental reasons. In that case, you should consider your entire utility bill and look at ways to be less dependent on utility companies.

Six Best Tankless Propane Gas & Electric Water Heaters

Whether you decide to select gas or electrically heated tankless water systems, the list below is based on the most highly-rated systems available from Amazon.com.

1. Rinnai RU Sensei Super

Rinnai RU Sensei Super at $1463.25 is a 9 GPM propane gas heated tankless water heater rated the best overall device. The system is fitted with a recirculation pump to reduce waiting times before the hot water is delivered to a minimum.

2. Rinnai V65eN Natural Gas Tankless Hot Water Heater

Rinnai V65eN Natural Gas Tankless Hot Water Heater at $583.20 is a 6.5 GPM product with excellent space-saving and energy efficiency. The device is regarded as the best value for money model.

3. Takagi T-H3-DV-N Condensing High-Efficiency

Takagi T-H3-DV-N Condensing High-Efficiency Natural Gas Indoor Tankless Water Heater at $1183.50 is a 10 GPM has a great cost to GPM ratio and is rated as a top-quality product.

4. Stiebel Eltron Tankless Water Heater (Tempra 24kW) Plus

Stiebel Eltron Tankless Water Heater (Tempra 24kW) Plus is an electrically powered on-demand hot water system for $698.00 and is equivalent to a 5 GPM gas-powered model.

5. Rheem 240V 2 Heating Chambers RTEX-18

Rheem 240V 2 Heating Chambers RTEX-18 is regarded as the Best Small Electric Tankless Heater at $359.  This model has two heating boilers and can generate of 18 kW. The maximum hot water flow is 4.4 GPM. That is usually enough for small and middle-sized households.

6. Stiebel Eltron DHC-E 12kW

Stiebel Eltron DHC-E 12kW is the smallest tankless water heater at $250 and is ideal for single point-of-use applications like a shower or kitchen basin.


Determining the size tankless water heater for your needs will require that you consider the inlet water temperature in your area and the maximum volume of hot water needed during your daily usage cycle. Consider the lowest winter temperatures of the supply water.

Also, consider how many people will be served with hot water and how many showers, faucets, and appliances can be run simultaneously at peak demand. The ease with which gas-powered systems can be installed in off-grid applications makes it preferable to electric-powered systems.

Use a consultant to help guide you in selecting the size of your tankless water heater system and advise you on how best to use and maintain such systems. (Related: Tankless Water Heaters: Advantages, Disadvantages & Costs) The high initial investment and installation costs require at least ten years to amortize.

Kenneth Wilson
November 23, 2021
Contractor Tips, Interior

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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