Electric Tankless Water Heater Size Calculator
Enter your specific requirements to determine the appropriate size for an electric tankless water heater.
Typical range: 40-60°F. Colder in northern climates, warmer in southern.
Standard household hot water is usually 105-120°F.
Simultaneous Fixture Usage (Estimate your peak demand):
Enter the number of fixtures you anticipate using at the same time. The calculator will sum their typical GPMs.
240V (Common for whole-house)
208V (Common in commercial/multi-family)
120V (Point-of-use only)
Higher voltage allows for more power (kW) and lower amperage for the same power.
Understanding Electric Tankless Water Heater Sizing
An electric tankless water heater provides hot water on demand, heating water only when you need it. Unlike traditional tank heaters, they don't store hot water, which can save energy. However, proper sizing is crucial to ensure you have enough hot water for your household's needs, especially during peak usage times.
Why Sizing Matters
An undersized tankless heater won't be able to keep up with demand, leading to lukewarm water or insufficient flow. An oversized unit might cost more upfront and could require significant electrical panel upgrades that aren't strictly necessary. The goal is to find the sweet spot that meets your hot water needs efficiently.
Key Factors in Sizing
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Incoming Water Temperature: This is perhaps the most critical factor. Water entering your home is colder in winter months and in northern climates. A tankless heater needs to work harder (require more power) to raise colder water to your desired temperature.
- Example: Raising 40°F water to 110°F requires a 70°F temperature rise. Raising 60°F water to 110°F only requires a 50°F rise.
- Desired Hot Water Temperature: Most people prefer hot water between 105°F and 120°F for showering and washing. Setting a lower desired temperature reduces the required temperature rise and thus the heater's power demand.
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Peak Flow Rate (GPM): This refers to the maximum amount of hot water (in Gallons Per Minute) you expect to use simultaneously. This is determined by how many hot water fixtures (showers, faucets, dishwashers, washing machines) might be running at the same time.
- Typical GPMs:
- Shower: 2.0 – 2.5 GPM
- Kitchen Faucet: 1.0 – 1.5 GPM
- Bathroom Faucet: 0.5 – 1.0 GPM
- Dishwasher: 1.0 – 1.5 GPM
- Washing Machine: 1.5 – 2.0 GPM
- Example: If you want to run one shower (2.5 GPM) and a kitchen faucet (1.5 GPM) simultaneously, your peak flow rate is 4.0 GPM.
- Typical GPMs:
- Electrical Voltage: Electric tankless heaters are typically available in 120V (for point-of-use, like a single sink), 208V, or 240V (for whole-house applications). Higher voltage allows the heater to deliver more power (kW) and achieve higher flow rates and temperature rises. It also impacts the required amperage, which is crucial for your home's electrical panel capacity.
How the Calculator Works
The calculator uses the following formula to determine the required power (Kilowatts or kW) for your tankless heater:
Required kW = Total GPM × Temperature Rise (ΔT) × 0.1466
Where:
- Total GPM: The sum of the flow rates of all fixtures you expect to use simultaneously.
- Temperature Rise (ΔT): The difference between your desired hot water temperature and the incoming cold water temperature.
- 0.1466: A conversion factor to translate GPM and temperature rise into kilowatts.
It also calculates the required amperage based on the chosen voltage, which is vital for ensuring your home's electrical system can support the heater.
Important Considerations
- Electrical Panel Capacity: Whole-house electric tankless water heaters often require significant amperage (e.g., 100-200 amps). Ensure your home's electrical panel has sufficient capacity and available breaker space. This may require an electrical service upgrade.
- Point-of-Use vs. Whole-House: For smaller demands or specific areas (like a distant bathroom or a single sink), a smaller point-of-use tankless heater might be more appropriate and require less electrical infrastructure.
- Professional Installation: Due to the electrical demands and plumbing requirements, it is highly recommended to have an electric tankless water heater installed by a qualified plumber and electrician.
Use this calculator as a guide to understand your needs. Always consult with a professional HVAC technician or plumber to confirm the best size and model for your specific home and local conditions.