Choosing The Correct Size Tankless Water Heater (For the Family of 2,3,4,5,6)

Correct Size Tankless Water Heater

People often get confused about the size of the tankless water heater because too small will be unable to fulfill your hot water requirement and too big will unnecessarily consume more energy, leading to high utility bills at the end of the billing cycle.

Size does matter when you plan to buy a tankless water heater. You need to consider the following two points before we start analyzing ways to measure the size requirement of the tankless water heater for your home.

The first thing to consider is the maximum hot water requirements and capacity of a heater to heat water in a minute. Heating capacity is measured in Gallon Per Minute.

Let us start with an estimate of the required water in a household at a given time. Research says we consume maximum hot water between 9 pm to 11 pm. During this time, people generally brush, shower, and run the dishwasher.

We have made a table including all the significant usage of the water in GPM. You can refer to this while deciding the size of the tankless water heater.

Place/MachinesGallon Per Minute (GPM)
Kitchen and Bathroom1.0 to 2.0 GPM
Shower2.0 to 3.0 GPM
Dishwasher1.5 to 2.0 GPM
Washing Machine2.0 to 2.5 GPM

According to this table, if you take a bath with 100 percent flow & 110 degrees F hot water and use two faucets simultaneously, your home needs a minimum of 5 GPM tankless water heater.

Generally, households require 5 to 10 GPM of water heater for their daily hot water needs. It is advisable to use electric tankless water heaters for requirements up to 8 GPM.

For higher needs, explore gas tankless water heaters. The price depends on the size. The bigger the water heater in terms of capacity, the higher the unit’s cost.

Inlet Water Temperature

The existing temperature of the water within the pipeline affects warming up the water significantly for example. Inlet water temperature is 77 degrees F in Southern Texas and 37 degrees F in Minnesota. Your tankless water heater needs to bridge this gap to heat water for your daily chores.

What Size Tankless Water Heater do I require for a Family of 2,3,4,5,6?

We will explain the required size of the tankless water heater according to the number of family members. Generally, a family with 2 or 3 family members requires a smaller unit larger families with four and above members need a bigger tankless water heater. We will find the exact GPM and kWh necessary for the best effect.

The size of the tankless water heater varies according to inlet temperature, hot water needs, and the region of your residence. Their requirements vary from North to South and cause different size needs.

We have made a table of estimates with a range of GPM and kW for the family size of 2,3,4,5,6.

Number of Family MembersGas Tankless Heater Size (GPM)Electric Tankless Heater Size (kW)
Family of 26 to 8 GPM10-18 kW
Family of 37 to 9 GPM15 to 23 kWh
Family of 48 to 10 GPM20 to 28 kWh
Family of 59 to 11 GPM25 to 34 kWh
Family of 611 plus GPM34 Plus kWh

These figures are for reference only. Practically, several essential factors like inflow water temperature and acute hot water needs determine the exact size of the tankless water heater you need.

Another significant consideration is the price factor. We will explain it through two examples to cover all the probable situations.

Example 1: What size of tankless water heater would be suitable to replace a 50-gallon unit?

You have a water heater with 30/40/80-gallon water capacity. You want to replace this with a tankless water heater.

Difference between a water tank and a tankless water heater

In the water tank, there is stored water, whereas a tankless water heater heats water when you need it. Generally, we consume 10 gallons of hot water during ten minutes of the bath. After that brush, dishwashing, and other activities, the three-member family can quickly consume 50 gallons of hot water.

This is entirely different from a tankless water heater. These HVAC units come with a powerful heat exchanger that warms up the water when you require it.

To replace the existing water heater of 50 gallons, you need a tankless gas heater with 10 GPM or a minimum of 27 kW of power supply in the Northern USA. In Southern USA, you need a 7 GPM gas tanker or 18 kW energy.

Tip: Always buy a little bigger tankless water heater than estimated to incorporate other deciding factors.

Example 2: What Size of Tankless Water Heater is suitable for a Family of Five members?

A family with five members can use multiple showers, taps, or faucets simultaneously. More people means more water consumption.

In a rough estimate, a 5-member family would require around 10 GPM gas tankless heater or 27 kW of power in the Northern USA. Your unit works harder to keep temperature up to 110 degrees F or 120 degrees F in these scenarios. 

In the Southern part of the country, consumption can decrease up to thirty percent. You will require a 7 GPM gas tankless water heater or consume 18 kW electricity.

Tip: You can use tankless water heater circulation pumps to reduce hot water wastage if you want water quickly from faucets and showers.

Tankless Water Heater Sizing Table

Family SizeCapacity Required(Measured in Gallon)
1-2 people30 Regular/ Low Usage, 40-50 High Usage30 Regular/ Low Usage, 40-50 High Usage
2-3 people40 Regular/ Low Usage, 50 High Usage40 Regular/ Low Usage, 40-50 High Usage
3-4 people50 Regular/ Low Usage, 80 High Usage50 Regular/ Low Usage, 50-75 High Usage
5+ people80 Regular/ Low Usage, 80+ High Usage50 Regular/ Low Usage, 75 High Usage

How Many Tankless Water Heaters Do You Need?

The number of tankless water heaters depends upon the size of the home. For standard homes and properties, one unit is enough. If you have a mansion, a big house with a long gallery, etc., you can try two tankless water heaters.


We hope our analysis of determining factors for the size of the tankless water heater will help your quest to find the best water heater for your home.

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