Rheem RTEX-18 240-Volt 2-Chamber 18kW Electric Tankless Water Heater

Choosing a Model

First, it is important to determine the incoming groundwater temperature where you live. This is significant because it represents the temperature that the water heater needs to elevate to meet your preferred water temperature.

Second, you must determine what you need to heat water for. For instance, is it just for a single sink, or is it for a 2-bathroom home? You can determine how much hot water you'll need based on the table below showing the average water usage of standard home fixtures.

Lastly, you can use the chart below to find the unit you need based on the groundwater temperature and the flow rate needed. On the red left side column find the incoming ground water temperature you found earlier. Next, look at the top dark grey column showing the GPM (Gallons Per Minute). Once you line both the column and row up you should see a number such as "8" or "24". This number indicates the number of kilowatts you need as well as the model number. All of these models start with "RTEX-", and the next portion will be the amount of kilowatts you need. For instance, a "RTEX-08" or a "RTEX-24"

If all else fails you can check out this simple guide below and choose based on the number of bathrooms you'll be supplying.