How to choose the right size of tankless water heater

Map of recommended tankless water heater models, based on incoming water temperatures
* Smallest recommended model is based on average winter ground water temperatures in the selected region and a flow rate of 3 gallons per minute. A larger tankless water heater than specified will provide greater flow rates. Incoming water temperatures may differ depending on water source, location, season, and climate. To make certain you are choosing the appropriate model, please measure your required flow rate and incoming water temperature.
Application Flow Rate in GPM at 60 PSI
Typical Shower 1.5 to 2.0 GPM
Typical Bath Tub Faucet 2.0 to 3.0 GPM
Bathroom Vanity Sink Faucet 0.5 to 1.5 GPM
Kitchen Sink Faucet 1.0 to 2.2 GPM
Clothes Washer 1.5 to 3.0 GPM
Choosing an appropriate model of electric tankless water heater for your home or business depends on 4 primary factors:

Incoming Water Temperature

The chart on the right is designed to offer you a general model recommendation based on the typical winter incoming water temperatures in various regions of North America. The model recommended on the chart for your area is based on a desired maximum flow rate of about 3 gallons per minute and should be regarded as the smallest recommended model for a general whole-house residential water heater - selecting a more powerful model will provide higher flow rates. Our chart and recommendations also assume that your home has enough electrical service to support the model suggested - see below.

Maximum Flow Rate Desired

Before making a decision on a tankless water heater model, you should consider the maximum flow rate of hot water you really need. Most people tend to over-estimate their simultaneous hot water demand. See the chart on the right for a few flow rate guidelines for various hot water applications in the home.

Most homes will only demand one major hot water use at a time, or a maximum of 2 flow-restricted showers, etc. Generally, a maximum flow rate of 3 GPM will be sufficient for such applications. You are encouraged to measure the flow rates at your showerhead and various outlets in the home to get a more definitive estimate of your true flow rate needs. In warmer climates, it is possible to use one of our more powerful models to run flow rates of up to 8 gallons per minute (depends on climate) and still achieve a comfortable output temperature suitable for a shower. However, in northern climates, your flow rate expectations should be significantly lower (up to 3.5 GPM), depending on your incoming water temperature.

The following chart illustrates the maximum flow rate that can be achieved with each model based on a 105F output temperature. Most people shower at a temperature of between 98 and 104F. This chart is based on a 240 volt input voltage and maximum flow rates are listed for various incoming water temperature levels. If your home or business has less than 240 volt power (208v), your maximum flow rate will be lower (please refer to the Stiebel Eltron Tempra brochure for flow rate data based on a 208 volt input).

Simply look up your incoming water temperature and desired flow rate to determine your model choice. Don't forget that incoming water temperature may change throughout the year depending on the season.

Incoming Water Temperature
Models 40F 45F 50F 55F 60F 65F 70F 75F 80F
Tempra 36 / Tempra 36 Plus 3.8 GPM 4.1 GPM 4.5 GPM 4.9 GPM 5.4 GPM 6.1 GPM 7.0 GPM 8.2 GPM 9.8 GPM
Tempra 29 /Tempra 29 Plus 3.0 GPM 3.3 GPM 3.6 GPM 3.9 GPM 4.4 GPM 4.9 GPM 5.6 GPM 6.5 GPM 7.8 GPM
Tempra 24 / Tempra 24 Plus 2.5 GPM 2.7 GPM 3.0 GPM 3.3 GPM 3.6 GPM 4.1 GPM 4.7 GPM 5.4 GPM 6.5 GPM
Tempra 20 / Tempra 20 Plus 2.0 GPM 2.2 GPM 2.4 GPM 2.6 GPM 2.9 GPM 3.3 GPM 3.7 GPM 4.4 GPM 5.2 GPM
Tempra 15 / Tempra 15 Plus 1.5 GPM 1.6 GPM 1.8 GPM 2.0 GPM 2.2 GPM 2.4 GPM 2.8 GPM 3.3 GPM 3.9 GPM
Tempra 12 / Tempra 12 Plus 1.3 GPM 1.4 GPM 1.5 GPM 1.6 GPM 1.8 GPM 2.0 GPM 2.3 GPM 2.7 GPM 3.3 GPM

Please note that your incoming water temperature could vary significantly from other homes in your immediate area, so checking your incoming water temperature is generally a more accurate means of determining which model is best suited to your home or business.

Desired Features

Stiebel Eltron presently offers two Tempra Series Models, the standard Tempra and the Tempra Plus. The primary difference between the 2 models is that the Tempra Plus models feature Stiebel Eltron's exclusive Advanced Flow Control technology. This technology automatically adjusts the flow of water when the unit's sensors detect that the flow demand is exceeding the heater's capacity. This feature is particularly useful in cold climates and other applications where the electrical service size does not permit the installation of a larger model and it may be desirable to handle occassional high flow demands. It is also desirable where high output temperatures are required for temperature critical applications (commercial dishwashers, etc.) where flow rates are not always predictable. All Tempra Plus Series models also feature a full digital temperature setting display.

Choosing which option is best for your needs is largely only a choice of personal preference and budget. Either model series is satisfactory for general residential applications.

Electrical Service

Your final consideration when selecting an electric tankless water heater is the electrical service to your home or business. All Stiebel Eltron tankless water heaters require 208 to 240 volts, single phase power. Since a tankless water heater works on demand, it must transfer a massive amount of heating power instantaneously to the water. Accordingly, tankless water heaters have a high peak electrical load even though they will reduce your overall electricity consumption substantially. Accordingly, consideration must be given to the number of AMPS of electrical service your home has to support the operation of the heater and other simultaneous electrical needs.

You can determine how many amps of electrical service your home has by inspecting the main breaker (the one you use to turn off power to the whole house) in your electrical panel.

Please see that chart below for important electrical specifications for each unit.

Model Recommended
Maximum Amperage
@ 240 V
Maximum Amperage
@ 208 V
Recommended Household
Electrical Service
Recommended Minimum
Wire Gauge
Tempra 36 / 36 Plus 3 x 50 AMPS * 150 AMPS 132 AMPS 300 AMPS 3 x #8 AWG*
Tempra 29 / 29 Plus 3 x 40 AMPS * 120 AMPS 105 AMPS 200 AMPS 3 x #8 AWG*
Tempra 24 / 24 Plus 2 x 50 AMPS * 100 AMPS 88 AMPS 150 AMPS 2 x #8 AWG*
Tempra 20 / 20 Plus 2 x 40 AMPS * 80 AMPS 70 AMPS 150 AMPS 2 x #8 AWG*
Tempra 15 / 15 Plus 2 x 30 AMPS * 60 AMPS 52 AMPS 100 AMPS 2 x #10 AWG*
Tempra 12 / 12 Plus 1 x 50 AMPS * 50 AMPS 44 AMPS 100 AMPS 1 x #8 AWG*

* Breaker size and wire gauge must meet all applicable local, state, provincial, and national electrical codes for your area - these are only guidelines that will apply to most installations. Some codes require use of electrical sub-panel for installation, especially when heater is not mounted within line-of-sight of the main electrical panel. Wiring should be sized to maintain a voltage drop of less than 3% under load. Please consult your electrician for more details.

The above specifications are only guidelines. If your home is already equipped with a large number of electrical appliances, you may require a larger electrical service than the one recommended above. Please consult an electrician before placing an order.

If your home does not have the minimum household electrical service as specified for your desired model, you can still purchase a Stiebel Eltron tankless water heater, however, upgrades to your electrical service will likely be required. It is recommended that you consult your certified electrician for a quote, prior to purchasing.

Please be aware that uninstalled units returned due to a home that has inadequate electrical service may subject to a re-stocking fee. Please see our POLICIES page for more details.