Electric Tankless Water Heater Size Calculator
Part of our Ventilation & Hot Water guides.
Estimate the electric tankless water heater size (kW) you need.
How it works
Heating water takes about 1 kW per 6.83 GPM·°F (1 BTU per pound per °F, 8.33 lb per gallon, 3,412 BTU per kWh). Groundwater in northern states can be near 40°F in winter, so check winter numbers.
Typical inputs
- Shower: 1.8-2.5 GPM (check your head).
- Bathroom faucet: about 0.5-1.2 GPM.
- Desired temperature: 105°F for showers is common.
- Incoming temperature: roughly 40-50°F in northern winters, 60-75°F in southern states.
Reading the result
The kW result is the heating power needed at that flow and rise. Choose a heater at or above that number. The amps figure shows the electrical current at 240 V, which helps your electrician check the panel.
Example
One 2.0 GPM shower and one sink in Chicago in winter: 2.5 GPM × 65°F ÷ 6.83 ≈ 24 kW, about 100 A. In Phoenix in summer the same use needs about 11 kW.
Limitations
This calculator gives an estimate; manufacturers' flow charts for specific models are the final word.
Frequently asked questions
What incoming temperature should I use?
Use your coldest-season groundwater temperature for a safe estimate.
What groundwater temperature should I use?
Use your coldest month. Northern states often see 40-50°F in winter; southern states 60-75°F. Your water utility may publish typical temperatures.
Is 105°F hot enough for a shower?
Most people shower at about 100-110°F. Using 105°F gives a realistic estimate; a higher set point needs more power.
Can I run two showers on one electric tankless unit?
Only if it's sized for both. Two 2 GPM showers in winter can require 35 kW or more, which many homes can't support electrically.
Does flow restrictor size matter?
Yes. A lower-flow shower head reduces the kW required and helps a smaller heater keep up.