Above about 150 A the unit is usually too big for a 200 A service, and gas or a tank makes more sense.
How the water heater size calculator works
Tank heaters are sized by their first-hour rating (FHR): how many gallons of hot water they deliver in one busy hour, starting with a full tank. WaterHeaterSize adds up your peak-hour demand with the US Department of Energy worksheet figures per use: shower 10 gal, bath 20, shaving 2, hand dishwashing or food prep 4, dishwasher 6 and clothes washer 7. Pick a heater whose FHR is at least that total. Tankless heaters are sized by flow and temperature rise instead: the flow of every fixture that may run at once (shower 2.0 GPM, bathroom faucet 1.0, kitchen faucet 1.5, tub 4.0, dishwasher 1.0, washer 2.0) times the rise from the incoming water to the setting. Heat needed = GPM × 500 × rise in °F, in BTU per hour; divide by efficiency for gas input, or by 3,412 for electric kW.
Tankless size by flow at a 70°F rise (50°F water heated to 120°F)
| Flow | Heat output | Gas input at 95% | Electric | Amps at 240 V |
|---|---|---|---|---|
| 1 GPM | 35,000 BTU/h | 36,842 BTU/h | 10.3 kW | 43 A |
| 2 GPM | 70,000 BTU/h | 73,684 BTU/h | 20.5 kW | 85 A |
| 3 GPM | 105,000 BTU/h | 110,526 BTU/h | 30.8 kW | 128 A |
| 4 GPM | 140,000 BTU/h | 147,368 BTU/h | 41 kW | 171 A |
| 5 GPM | 175,000 BTU/h | 184,211 BTU/h | 51.3 kW | 214 A |
| 6 GPM | 210,000 BTU/h | 221,053 BTU/h | 61.5 kW | 256 A |
Groundwater is colder in northern climates in winter (often 40°F or less), which raises the rise and the size you need. Low-flow fixtures reduce it. Gas units need correctly sized gas lines and venting, and electric units need a dedicated circuit; have a licensed installer confirm the final choice. Results are estimates for planning.
Frequently asked questions
How many amps does an electric tankless water heater need?
kW × 1,000 ÷ 240. A 27 kW whole-house unit draws about 113 A; a 36 kW unit about 150 A.
Can an electric tankless heater run two showers?
In warm climates, yes. With 50°F water heated to 120°F, two 2 GPM showers need about 41 kW and 171 A.
How efficient are electric tankless heaters?
Close to 100% at the point of use, with no standby loss, though electricity often costs more than gas per BTU.
Is WaterHeaterSize free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.