Electric Tankless Water Heater Sizing Calculator

Choosing the right electric tankless water heater for your home is essential to ensure consistent hot water without overloading your electrical system. This electric tankless water heater sizing calculator helps you estimate the ideal heater size based on your household’s hot water usage, incoming water temperature, and electrical voltage.

Just enter how many devices (like showers or sinks) you expect to use at once, your local incoming water temperature, and your desired hot water temperature. Then select your home’s voltage (208V or 240V). The tool will recommend the minimum power rating (in kilowatts) your electric tankless water heater should have — and even estimate how many amps it will draw, helping you plan for safe installation.

How to Use the Electric Tankless Water Heater Sizing Calculator?

Step 1: Enter the Number of Devices Using Hot Water Simultaneously

Begin by estimating how many hot water fixtures or appliances might be running at the same time in your household. This includes showers, faucets, washing machines, and dishwashers. For example, if you might run two showers and a sink at the same time, enter “3”. This number helps the calculator determine your total water demand in gallons per minute (GPM).

Step 2: Input the Incoming Water Temperature

Next, enter the temperature of the cold water entering your home. This varies depending on your location and season. In colder climates, incoming water might be as low as 40–50°F, while warmer areas may have incoming water closer to 60–70°F. If you’re unsure, 50°F is a safe general estimate.

Step 3: Set Your Desired Hot Water Temperature

Most households prefer hot water around 120°F, but you can adjust this to your comfort level. This value represents how hot you want the water to be at the tap. The calculator uses the difference between this value and your incoming temperature to determine the required heating performance.

Step 4: Select Your Electrical Voltage

Choose between 208V or 240V, depending on your home’s electrical setup. Most residential homes in the U.S. use 240V, but some buildings (especially older apartments or commercial settings) may use 208V. Selecting the correct voltage helps estimate the required amperage draw and ensures compatibility with your electrical panel.

Step 5: Click “Calculate Size” to View Results

Once all fields are completed, press the Calculate Size button. The tool will instantly display:

• The recommended minimum heater power (in kilowatts, kW)
• An estimate of the amperage the heater will draw at the selected voltage

Use this result to choose a unit that meets or exceeds your requirements while matching your home’s electrical capabilities.

Frequently Asked Questions (FAQ)

What happens if I install a heater that’s too small?

If the heater is undersized, it won’t be able to keep up with your hot water demand. This often leads to lukewarm water, inconsistent temperatures, or the need to stagger hot water use between fixtures. A properly sized heater ensures all devices receive hot water without interruption.

Is it okay to choose a heater that’s oversized?

While oversizing is generally safe, it can be inefficient and more expensive up front. An oversized electric heater may also require more amps than your breaker panel supports. It’s best to choose a size that closely matches your actual demand to balance performance and efficiency.

How is the heater power (kW) calculated?

The calculator uses a formula based on your total flow rate, the temperature rise (difference between incoming and desired temperatures), and a conversion factor that accounts for the heat capacity of water. This gives you the minimum kilowatts needed to instantly heat water at your required rate.

What does the amperage estimate mean?

Amperage tells you how much electrical current the unit will draw. This helps ensure your home’s electrical panel and wiring can support the heater. For example, a 24kW heater on 240V may draw around 100 amps — which might require a dedicated circuit or panel upgrade.

Can I use this tool outside the U.S.?

Yes, but keep in mind that it uses Fahrenheit for temperature and assumes voltages of 208V or 240V, common in the U.S. If your country uses different voltages (like 220V or 380V), the amperage calculation may need adjustment.

What if my fixtures have low or high flow rates?

The calculator assumes an average flow rate of 2.0 GPM per device. If you’re using low-flow fixtures or high-demand appliances (like soaking tubs or commercial dishwashers), you may want to adjust the number of devices accordingly to reflect your real water usage.

Is this tool suitable for commercial installations?

This tool is primarily designed for residential use. For commercial setups with large or variable demands, it’s best to consult with a plumber or manufacturer to size equipment based on detailed peak load calculations.

Check out our reviews of the best tankless water heaters below:

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• Best RV Tankless Water Heate