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Instantaneous Water Heater Calculator

This calculator helps you determine the required power of an instantaneous water heater or estimate the achievable water flow or temperature based on the heater’s power. It’s useful for engineers, plumbers, and homeowners planning water heating systems. Just input the desired temperature, cold water inlet temperature, and power or flow rate — and get instant results.

Instantaneous Water Heating Performance Calculator

Input Fields
P
kW
Tc
°C
Th
°C
If enabled, the result will update automatically when you change any value.

Water Heater Power, Flow, and Temperature Formulas

Formula
\[\text{Performance (L/min)} = \frac{P}{(T_h – T_c) \times 0.073}\]\[P = Q \times (T_h – T_c) \times 0.073\]\[T_h = \frac{P + Q \times 0.073 \times T_c}{Q \times 0.073}\]

Where:

  • $$P$$ is the power of the heater in kilowatts (kW)
  • $$T_h$$ is the target hot water temperature (°C)
  • $$T_c$$ is the cold water inlet temperature (°C)
  • $$Q$$ is the flow rate in liters per minute (L/min)
  • 0.073 is a constant derived from water heat capacity and unit conversion


These formulas are commonly used in the design and selection of electric tankless (instantaneous) water heaters. By rearranging the equations, you can calculate:

  • the required power based on desired flow and temperature difference
  • the achievable flow rate for a given power
  • the hot water temperature based on input conditions

This calculator supports efficient planning for residential and industrial hot water supply, especially in energy-optimized systems.

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One thought on “Instantaneous Water Heater Calculator

  1. Maintaining your water heater is definitely important. I am going to check mine as soon as I get home. I will evaluate any problems that I can find. Perhaps I will find something amiss. Do you think duct tape would seal cracks in the side?