Electric Tankless Water Heater Calculator

Why Sizing Matters for Tankless Water Heaters

Unlike traditional tank water heaters, tankless models heat water on demand. That means they must supply enough hot water instantly, at the right flow rate, without running out.

If your unit is too small:

• Showers may run lukewarm in winter.
• Running two fixtures at once could drop temperatures suddenly.
• The system will overwork and may fail sooner.

If your unit is too large:

• You’ll spend extra on installation and energy capacity.
• Your home’s electrical service may not support it.

A properly sized tankless heater balances comfort, efficiency, and cost.

How the Electric Tankless Water Heater Calculator Works

The calculator uses three main inputs:

1. Incoming Water Temperature (Tin):
   This depends on where you live. Northern states may see winter groundwater at 40–50°F, while southern states can average 65–75°F.
2. Desired Output Temperature (Tout):
   Most households prefer shower and sink water around 105°F. Some may set it higher for dishwashing or sanitization.
3. Simultaneous Hot Water Demand (GPM):
   GPM (gallons per minute) measures flow. A low-flow faucet may use 1.0 GPM, while an average shower uses 2.5 GPM. Multiple fixtures running together add up quickly.

The Formula Behind the Calculator

The tool applies a simple but effective sizing formula:

[
kW \approx GPM \times \Delta T \times 0.0024
]

Where:

• ΔT (Temperature Rise) = Tout − Tin
• GPM (Gallons Per Minute) = household hot water demand
• 0.0024 = conversion factor for water heating power

This tells you the minimum kW required for your heater to deliver consistent hot water.

Example: Using the Calculator

Let’s say you enter:

• Incoming Temperature (Tin): 50°F
• Desired Temperature (Tout): 105°F
• Hot Water Demand: 3.5 GPM (average home scenario)

Step 1: Calculate ΔT = 105 − 50 = 55°F
Step 2: Multiply → 3.5 × 55 × 0.0024 ≈ 0.462 kW
Step 3: Convert → The calculator rounds up, giving 19 kW minimum required power.

This means you’ll need an electric tankless water heater rated at 19 kW or higher.

How to Use the Calculator (Step by Step)

1. Enter your incoming water temperature.
   (Tip: Search your state’s average groundwater temperature if unsure.)
2. Enter your desired hot water temperature.
   (Most homes use 105°F as the standard.)
3. Select your simultaneous hot water demand scenario:
   • 1.0 GPM → Small faucet or sink
   • 2.5 GPM → Single low-flow shower
   • 3.5 GPM → Average home
   • 5.0 GPM → Two showers running
   • 6.5 GPM → Large home / multiple fixtures
4. Hit Calculate.
   The results show:
   • Required temperature rise (ΔT)
   • Calculated GPM demand
   • Minimum required heater power in kW

Benefits of Using the Electric Tankless Water Heater Calculator

• Precision: No more guessing what size heater you need.
• Energy savings: Avoid overspending on a unit too large for your needs.
• Comfort: Prevent cold surprises when multiple taps are in use.
• Planning aid: Helps check if your home’s electrical system can support the required kW.

Important Notes

• The calculator shows minimum kW requirements. Always check the manufacturer’s specifications before purchasing.
• Electrical service limits matter. Larger tankless units may require 200-amp service or dedicated breakers.
• In colder regions, you may need a higher-rated unit to handle winter groundwater temperatures.