Electric Tankless Water Heaters

Electric tankless units are simple to vent (they don't) and hard to wire. A 27 kW model draws about 112 amps at 240 volts, split across several double-pole breakers; a 36 kW unit draws 150 amps. Many 200-amp panels can't spare that once heating and cooking loads are counted.

Flow is limited by power. Each kilowatt lifts about 0.14 GPM by 50 degrees, so a 27 kW unit holds roughly 3.7 GPM at a 50-degree rise but under 3 GPM at a 65-degree winter rise. That's why electric whole-house units do best where inlet water stays mild.

On 208-volt service a 240-volt element delivers about 75% of its rated power. The calculator shows amps at your voltage; a licensed electrician confirms the panel and wire.

Start from your hot-water demand

Install facts to confirm first

• Count the amps first. Divide the watts by your voltage: a 27 kW unit is about 112 amps at 240 volts. A licensed electrician confirms the panel and wire.
• Cold inlet water cuts flow. Each kilowatt lifts about 0.14 GPM by 50 degrees. At a 70-degree winter rise, flow drops by close to 30% against a 50-degree rise.

Full electric tankless water heater spec grid

Figures are the makers' own, quoted with their conditions on each product page. Prices change; each product page links to the current price.