To size an electric tankless water heater, consider both flow rate (how much hot water you need) and temperature rise (the difference between incoming and desired water temperature). Next, estimate the highest flow rate you'll have running simultaneously (e.g., 1 sink + 1 shower), and factor in your climate's cold water temperature and desired hot water temperature to determine the required temperature rise. In most cases, 105F output temp is considered optimal, and tempers abode 105 could cause scalding or irritations of the skin. Last, add your finding (e.g., 2.5 x 33 x 147= 12,127.5) or GPM x temp rise x 147= KW) and select a heater with enough kW output to achieve.
Below is a list of hot water faucets in the home and the gallons per minute they emit:
- Shower head: 1.5 - 2.0 gpm
- Bathtub: 2.5 - 3.5 gpm
- Kitchen sink: 0.75 - 1.5 gpm
- Hand sink:0.5 - 1.0 gpm