Electric Tankless Water Heater Sizing Calculator

Determine the ideal size for your electric tankless water heater based on your hot water needs and electrical capacity.

How to Size an Electric Tankless Water Heater

Electric tankless water heaters provide on-demand hot water without the energy losses associated with storage tanks, but proper sizing is crucial for performance. Unlike gas models that measure capacity in BTUs, electric units are sized in kilowatts (kW). The key factors in sizing include your simultaneous hot water demand (flow rate in gallons per minute), the temperature rise needed (difference between incoming and desired water temperature), and your home's electrical capacity.

Understanding Electric Tankless Water Heater Requirements

Electric tankless water heaters require significant electrical power to instantly heat water as it flows through the unit. While they're more energy efficient than tank heaters (no standby losses), they demand much higher instantaneous power. A typical residential electric tankless unit requires between 18-36 kW, which translates to 75-150 amps at 240V.

Calculating Flow Rate (GPM) Needs

Your total flow rate is the sum of all hot water fixtures that might run simultaneously. Common fixture flow rates:

  • Standard shower: 1.5-2.5 GPM (with low-flow heads)
  • Kitchen faucet: 1.0-1.5 GPM
  • Bathroom faucet: 0.5-1.0 GPM
  • Washing machine: 1.5-2.5 GPM
  • Dishwasher: 1.0-2.0 GPM

For example, if you might run two showers (2.0 GPM each) and a kitchen faucet (1.5 GPM) simultaneously, your peak demand is 5.5 GPM.

Determining Temperature Rise

Temperature rise is calculated by subtracting your incoming water temperature from your desired output temperature. Incoming water temperature varies by region:

  • Northern states: 35-45°F
  • Central states: 45-55°F
  • Southern states: 55-70°F

For a desired 120°F output with 50°F incoming water, you need a 70°F rise. The greater the temperature rise required, the more powerful your unit needs to be.

Power Calculation Formula

The power required (in kW) is calculated using:

kW = (GPM × 8.34 × ΔT) / 3412

Where:

  • GPM = Total flow rate (gallons per minute)
  • 8.34 = Weight of one gallon of water (lbs)
  • ΔT = Temperature rise needed (°F)
  • 3412 = Conversion factor (BTU to kWh)

For example, to heat 3 GPM with a 60°F rise: (3 × 8.34 × 60) / 3412 = 4.4 kW

Electrical Requirements

Electric tankless water heaters have significant electrical demands:

  • Voltage: Most residential units require 240V, though some point-of-use models use 120V
  • Amperage: A 27 kW unit at 240V draws 112.5 amps (27,000W ÷ 240V)
  • Circuit Breaker: Typically requires a double-pole breaker sized at 125% of the unit's amp draw
  • Wire Size: Depends on amperage - often 6 AWG or larger for whole-house units

Whole-House vs Point-of-Use Installation

Whole-house systems require high-capacity units (typically 27-36 kW) and may need electrical panel upgrades. They can service multiple fixtures but struggle in very cold climates or with high simultaneous demand.

Point-of-use systems are smaller (3-12 kW) and installed near a single fixture (like under a kitchen sink). They're ideal for:

  • Remote bathrooms far from main water heater
  • Boosting temperature for specific fixtures
  • Homes with limited electrical capacity for whole-house units

Advantages of Electric Tankless Water Heaters

Pros

  • Energy savings (no standby heat loss)
  • Compact size saves space
  • Precise temperature control
  • Long lifespan (20+ years)
  • No venting required
  • No risk of gas leaks or carbon monoxide

Cons

  • High upfront cost
  • May require electrical service upgrade
  • Limited simultaneous use in cold climates
  • Higher operating costs than gas in many areas
  • Can be affected by power outages

Installation Considerations

Proper installation is critical for electric tankless water heaters:

  • Electrical Service: Most homes need at least 200 amp service for whole-house units
  • Wiring: Requires dedicated circuit with proper wire gauge
  • Location: Install as close as possible to points of use to reduce heat loss in pipes
  • Hard Water: Consider water softening if you have hard water to prevent scale buildup
  • Professional Installation: Highly recommended due to complex electrical requirements

Frequently Asked Questions

Can an electric tankless water heater service a whole house?

Yes, but only if properly sized for your peak demand and climate. In cold climates with high demand, multiple units or a hybrid approach may be better.

How many amps does a typical electric tankless water heater need?

Whole-house units typically require 80-150 amps at 240V, often needing a dedicated subpanel.

Are electric tankless water heaters more efficient than gas?

They're more energy efficient (no standby losses, nearly 100% energy conversion), but may cost more to operate depending on local electricity vs gas prices.

Can I install an electric tankless water heater myself?

While possible for point-of-use units, whole-house installations should be done by licensed electricians due to high-voltage requirements and potential need for panel upgrades.

Why does my electric tankless water heater deliver lukewarm water?

This usually indicates undersizing - either the unit can't handle your flow rate or the temperature rise needed for your climate.

Conclusion

Electric tankless water heaters offer excellent energy efficiency and endless hot water when properly sized. Use our calculator above to determine your kW requirements based on your flow rate needs and temperature rise. Remember that electrical capacity is often the limiting factor - consult with a qualified electrician to assess your home's ability to support an electric tankless system before purchasing.