Tankless Water Heater Size
Count what runs at the same time, at the coldest time of year.
Hot water demand
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Estimates only, from the DOE sizing method (add up flow, find the rise). Fixture flows are typical; check yours.
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How tankless sizing works
The DOE method: add up the flow of the fixtures you'd use at the same time, then work out the temperature rise (output temperature minus incoming water). Heating water takes about 500 BTU/h for each gallon per minute per °F of rise, so heat needed = 500 × gpm × rise. Electric kW = BTU/h ÷ 3,412; amps = watts ÷ 240. A gas unit's input must be higher than the heat delivered by its efficiency: about 95% for condensing units, about 82% for standard (non-condensing) ones.
Typical flows: showers 1.5–2.5 gpm (federal maximum 2.5, WaterSense 2.0 or less), bathroom faucet 0.5–1.0, kitchen faucet about 1.5, dishwasher 1–2, washer 1.5–3, tub filler about 4 or more.
What size tankless water heater do I need for two showers?
Two 2.0 gpm showers need 4 gpm. With 50°F incoming water heated to 120°F (a 70°F rise) that is 140,000 BTU/h of heat: about 147,000 BTU/h input for a 95% efficient gas unit, or 41 kW (about 171 A at 240 V) for an electric one.
Can my electrical panel handle an electric tankless water heater?
Often not for a whole house. Whole-home electric units draw 100–150 A on two to four dedicated circuits, which many 200 A panels can't spare alongside a range, dryer and AC. Have a licensed electrician do a load calculation; a gas tankless or a heat pump water heater may suit better.
How do I find my incoming water temperature?
Run the cold tap for a minute in winter and measure it with a kitchen thermometer. Northern homes often see around 40°F, southern ones 60–70°F. Size for the coldest month. The region picks above fill in rough winter values: far north 40°F, north 45°F, US average 50°F, mid-South 55°F, South 60°F, Gulf Coast, Florida and desert Southwest 70°F.
Sources: DOE Energy Saver: sizing a new water heater; PNNL instantaneous water heater sizing; EPA WaterSense.