Electric Tankless Water Heater Size Calculator
Find the right electric tankless water heater size by entering your hot water demand and temperature rise details below.
List of the Best Electric Tankless Water Heater:
Why Use an Electric Tankless Water Heater Size Calculator?
Choosing the right electric tankless water heater is crucial for ensuring your home has a consistent and efficient hot water supply. An undersized unit may lead to cold showers and discomfort, while an oversized one wastes energy and increases costs. Our electric tankless water heater size calculator helps you determine the exact size you need by factoring in flow rate (GPM) and temperature rise.
How Does the Calculator Work?
The electric tankless water heater size calculator estimates the required kW (kilowatt) output based on how many fixtures you plan to use at the same time and the difference between your incoming water temperature and your desired output temperature. This sizing ensures that your heater delivers enough energy to instantly heat water without delay or shortage.
Understanding Flow Rate and Temperature Rise
Flow rate is measured in gallons per minute (GPM) and represents how much water you’ll need at once. For example:
- Shower: 1.5 – 2.5 GPM
- Sink: 0.5 – 1.5 GPM
- Washing Machine: 1.0 – 2.0 GPM
Temperature rise is the difference between the incoming cold water temperature and the temperature you want at your taps. If your groundwater is 50°F and you want a hot shower at 120°F, your required temperature rise is 70°F.
How to Use the Electric Tankless Water Heater Size Calculator
- Count the number of fixtures you expect to use simultaneously.
- Add up their total flow rate (you can refer to standard GPM values).
- Input the average temperature of your incoming water.
- Enter your desired output water temperature (typically 120°F).
- Click “Calculate” to get your required kW output.
Why Sizing Accuracy Matters
Proper sizing ensures your water heater can meet demand during peak usage without straining the unit or causing dips in water temperature. Undersized units struggle to maintain temperature and often lead to dissatisfaction. Oversized models increase your electric bill unnecessarily and take up more space.
Sample Sizing Scenarios
| Usage Scenario | Total GPM | Temp Rise (°F) | Recommended kW |
|---|---|---|---|
| 1 Shower | 2.0 | 70 | 20.7 kW |
| 1 Shower + 1 Sink | 3.0 | 60 | 26.6 kW |
| 2 Showers + 1 Washing Machine | 5.5 | 65 | 52.8 kW |
Electric vs. Gas Tankless Water Heaters
Electric models are compact, more energy-efficient for small to medium households, and easier to install. They’re ideal for homes with limited space and electric-only setups. Gas models are more powerful but require venting and access to natural gas or propane. Our electric tankless water heater size calculator is designed for homes using electricity as their primary power source.
Choosing the Right Unit
Once you calculate your required kW output, you can compare it with product specifications. Some popular electric tankless water heater models include:
- EcoSmart ECO 27 – 27 kW, suitable for up to 6.5 GPM in warm climates.
- Stiebel Eltron Tempra 24 Plus – 24 kW, excellent flow control and reliability.
- Rheem RTEX-18 – 18 kW, compact and ideal for point-of-use or small homes.
Efficiency and Energy Tips
- Install low-flow showerheads and faucets to reduce total GPM.
- Lower your target water temperature slightly to reduce power usage.
- Insulate pipes and reduce the distance from heater to fixtures.
- Consider installing separate units for different parts of your home (e.g. bathroom and kitchen).
FAQs About Tankless Water Heater Sizing
What happens if I choose a water heater that’s too small?
You may experience cold water mid-shower, or it may struggle to deliver hot water to multiple taps at once.
Can I oversize my unit just in case?
While slightly oversizing is fine, a much too large unit may waste electricity and drive up installation costs.
What if I live in a cold climate?
In colder climates with low groundwater temperatures, you’ll need a higher kW heater to achieve the same output temperature. Our calculator adjusts for this by factoring in temperature rise.
Is electric tankless water heating right for me?
If your household uses moderate hot water, has access to reliable electricity, and prefers compact installations, then yes—especially when sized using our calculator.
Final Thoughts
Using an electric tankless water heater size calculator is the most efficient way to avoid guesswork and ensure long-term satisfaction with your hot water system. Whether you're building a new home or upgrading your water heater, make sizing your first step for success. It saves energy, improves comfort, and ensures your home runs smoothly, one hot shower at a time.