Electric Tankless Water Heater Sizing Calculator

Required kW is calculated as: kW = (Peak GPM × Temperature Rise in °F) / 6.824. For a 2-shower home in a moderate climate (4.0 GPM at a 50°F rise), you need approximately 29.3 kW (typically handled by a 27 kW or 36 kW unit). In cold northern climates, whole-house electric units usually require 36 kW.

At standard 240V single-phase residential power, an 18 kW unit draws 75 Amps (requires two 40A breakers), a 27 kW unit draws 112.5 Amps (requires three 40A breakers), and a 36 kW unit draws 150 Amps (requires four 40A breakers).

A 200-Amp panel can usually handle up to a 24 kW or 27 kW unit if other major household electric loads (like central AC, electric oven, EV charger) are balanced. A 36 kW unit drawing 150A often requires a 300A or 400A service upgrade.

Electric tankless heaters have a thermal efficiency of 98% to 99%, meaning virtually all electrical energy consumed is transferred directly into the water with minimal standby heat loss.

Northern winter groundwater can drop to 37°F–40°F. Heating that water to 120°F requires an 80°F–83°F temperature rise. An electric unit producing 27 kW can only produce ~2.3 GPM under an 80°F rise, which supports only a single shower head at a time.

Each 40-Amp double-pole circuit typically requires 8 AWG copper wire, while 50-Amp circuits require 6 AWG copper wire. Multi-breaker units require running multiple separate copper cable sets from the breaker panel to the heater.