Can an Electric Tankless Water Heater Run a Whole House?


Many people are starting to use electric tankless water heaters in their house. But, they are unsure if an electric tankless water heater can provide sufficient hot water for a whole house. So, I did some research.

An electric tankless water heater can run a whole house given that the cold water temperature is 57°F (14°C) and above. A 36 kW electric tankless water heater can provide adequate hot water for three persons simultaneously.

In order for an electric tankless water heater to run a whole house, it must be sized carefully. Otherwise, people can't take a shower at the same time.

How to Use an Electric Tankless Water Heater to Run a Whole House?

The ability of an electric tankless water heater to produce sufficient hot water is depending on two factors; a) water flow rate and b) cold water temperature.

Water flow rate is pretty much fixed by plumbing fixtures and governed by regulations to control water usage. However, cold water temperature fluctuates and it is affecting the sizing of tankless water heaters.

Sizing of Electric Tankless Water Heaters

The colder the water temperature going into the water heater, the lesser the hot water produced by the water heater. In other words, tankless water heaters raise the temperature of the cold water by a fixed degree.

So, if your house is located in the northern region where the cold water temperature is very low, a single unit of electric tankless water heater is most likely not enough for your house.

See the below map for the average cold water temperature in the United States:

Cold Water Temperature Map Derived from EcoSmart

To find out if one unit of electric tankless water heater is sufficient for your house, we need to assume that there are three persons in the house who will likely take a shower at the same time.

So, the water heater needs to cater to the hot water flow rate for three shower heads simultaneously.

Generally, each shower head has a water flow rate of between 1.5 GPM to 2.5 GPM. Thus, we can say that as long as an electric tankless water heater has a water flow rate of around 6 GPM, it is good enough to run a whole house where three persons will shower simultaneously.

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Below is the minimum electric tankless water heater size required to run a whole house based on different cold water temperatures:

Cold Water TemperatureElectric Tankless Water Heater Size
57°F36 kW
62°F36 kW
67°F36 kW
72°F27 kW
77°F24 kW
Minimum Electric Tankless Water Heater Size for 3 Persons Showering Simultaneously

From the above table, the higher the cold water temperature, the lower the capacity of the electric tankless water heater.

Remember that this sizing is based on three persons showering simultaneously.

So, if your family members don't necessarily need to shower at the same time, you have the chance to reduce the capacity of the electric tankless water heater or be able to use an electric tankless water heater for the entire house even when the cold water temperature is below 57°F (14°C).

For two persons showering simultaneously, an electric tankless water heater needs to have around 5 GPM of water flow rate.

Below is the minimum electric tankless water heater size required for 2 persons showering simultaneously based on different cold water temperatures:

Cold Water TemperatureElectric Tankless Water Heater Size
52°F36 kW
57°F36 kW
62°F36 kW
67°F27 kW
72°F24 kW
77°F24 kW
Minimum Electric Tankless Water Heater Size for 2 Persons Showering Simultaneously

As you can see, if there are only two persons showering simultaneously, you can use a smaller size electric tankless water heater to achieve the same objective.

Hot Water Pipe Insulation

When you're using only one electric tankless water heater for the entire house, the hot water pipe is likely to be longer than if you would use multiple electric tankless water heaters.

Furthermore, a 36 kW electric tankless water heater is just enough for three persons to shower at the same time. If the temperature of the hot water drops too much when traveling from the water heater to the respective shower head, it's not going to be a good shower experience.

Hence, you should consider insulating your hot water pipes, especially if they are made of copper or galvanized steel that has a high thermal conductivity.

If your hot water pipes are made of plastic materials such as CPVC, PPR or PEX, thermal insulations may not be necessary but it is recommended in order to maintain performance and reduce energy loss.

What is the Downside of Electric Tankless Water Heaters?

Electric tankless water heaters are prone to hot water temperature fluctuations. Since they are dependent on the water flow rate and cold water temperature, any changes will result in a change in the hot water temperature.

Perhaps, the biggest downside of electric tankless water heaters is the consequences of undersized units. If you got yourself an undersized electric tankless water heater, you are not able to raise the temperature of the hot water unless you are willing to reduce the hot water flow rate, which in turn affects your shower experience.

The second problem with electric tankless water heaters is the high running current. A 36 kW electric tankless water heater draws about 150 A of electric current. If your house was not designed for that, you'll need to pay extra for the upgrade.

Should You Use an Electric Tankless Water Heater to Run a Whole House?

In conclusion, should you use an electric tankless water heater to run a whole house? If you ask me, I'll let you proceed with it if you're staying in the southern region where the cold water temperature is relatively higher.

However, if you think that the cold water temperature in your house (including during the winter) is quite low, I'll suggest you get multiple electric tankless water heaters or one unit for each bathroom to secure adequate hot water.

Overall, I don't recommend people use an electric tankless water heater to provide hot water for multiple bathrooms because the hot water pipes are going to be longer and thus, energy loss is greater. I always recommend people to use one electric tankless water heater for one bathroom or two bathrooms if they are built close to each other.


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