Formulas
- Electric kW = GPM x temperature rise (F) / 6.83
- Gas input BTU/h = GPM x 500 x temperature rise / 0.95
The 6.83 divisor is 3,412 BTU per kWh divided by about 500 BTU per gallon per degree (8.34 lb per gallon times 60 minutes). The gas line uses the same 500 and then divides by 95 percent efficiency. The amp figure is kilowatts times 1,000, divided by 240 volts, rounded up. It is not a breaker size. Continuous loads need extra circuit headroom, which an electrician calculates.
Worked example
One shower and one faucet are 3.0 GPM. From 50 F to 120 F the rise is 70 F. Electric size is 3 x 70 / 6.83 = 30.7 kW. Two showers alone, 4.0 GPM at the same 70 F rise, are about 41 kW. That is the whole-house electric case called out above.
Tank versus tankless
A storage heater is sized by first-hour rating, not by this flow math. A household of one or two people often looks at a 30 to 40 gallon first-hour rating, three or four people at 40 to 60, and larger households above that. Tankless units do not run out in the same way, but they cannot exceed the flow you size here. If two showers and a dishwasher really run together, size for that peak or plan on two heaters.
Gas venting, gas pipe size, and electric service are installation questions. Have a licensed plumber or electrician confirm the unit before you buy one that the house cannot supply.
Frequently asked questions
How do I calculate a tankless water heater size?
Add the gallons per minute of the fixtures you run together, then subtract incoming water temperature from the temperature you want. Electric kilowatts = GPM x rise in F / 6.83. Gas input BTU per hour = GPM x 500 x rise / efficiency.
What flow rates does this page use?
Shower 2.0 GPM, faucet 1.0 GPM, dishwasher 1.5 GPM, washing machine 2.0 GPM. If your shower head is labeled 1.5 or 2.5 GPM, the total will differ. The formula still applies to whatever GPM you have.
Why is whole-house electric tankless so large?
Two showers at 2.0 GPM each, plus the default math at a 70 F rise, is 4.0 GPM and about 41 kW. Many homes cannot add that load. Gas, or a heat-pump tank, is the usual whole-house choice. A single bathroom can be a smaller electric unit.
What incoming water temperature should I use?
Groundwater is often about 40 to 50 F in the northern United States and about 60 to 70 F in the south. Use the cold-tap temperature on a winter morning if you can measure it. A colder inlet means a larger heater.