Instantaneous Water Heater Performance

The relationship between flow rate (litres per minute), available heating power (measured in kW) and water temperature is given by the following formula:

Power in kW to Heat Water = [ Litres x Temp Rise oC] / [ Time in Minutes x 14.3]

Ideally the flow rate of water at the required temperature would be as high as possible but in practice it is limited by the available heating power.




(Note that the actual temperature depends on the incoming mains water temperature which varies according to the season but is typically 5oC in winter and 10oC in summer. (To calculate output temperature simply add the temperature of the incoming water (oC) to the temperature rise generated by the water heater, for example if the incoming water temperature is 5oC and the heater is able to increase the water temperature by 40oC then the outlet temperature will be 45oC) . 

Power (kW)
Temperature Increase (degs C) 
 Flow Rate (L/min) 


Stored Water Reheat Time  (Recovery Time)

The following table shows the time taken for water to be reheated (assuming a nominal temperature rise of 50oC) for a range of storage capacities and heating powers. Ideally the reheat time would be as short as possible but in practice it is determined by the size of the tank and the available heating power.

 Time in Minutes to Heat Water = [Litres x Temp Rise oC] / [kW Loading x 14.3]


Volume (L)
Temp Rise (°C)
Reheat Time (Mins) 





Temperature in Celcius = [ oF -32 ] x 5/9

Temperature in Fahrenheit = [ oC x 9/5] + 32


1 Gallon = 4.54 Litres

1 Litre = 0.22 gallons

1 m3 = 1000 Litres


1m = 3.28ft = 39.4in


1lb = 0.45kg

1 kg = 2.21 lb


1 bar = 14.5 psi = 100kNm-2 = 10.2 m head of water

1m head of water = 9.8 kNm-2


1 kcal is the heat required to raise 1 kg of water by 1oC

1 BTU is the heat required to raise 1lb of water by 1 oF

1 kcal = 4187 Joules = 3.97 British Thermal Units (BTU)

1 BTU = 17.6 Watt Minutes = 0.252 kcal

1 kW Hour = 3214 BTU = 860 kcal

Calorific Value of Propane = 50,000 kJ/kg = 13.9 kWh /kg

Calorific Value of Butane = 49,000 kJ/kg = 13.1 kWh /kg

Propane boiling point at atmospheric pressure = -42 ºC

Butane boiling point at atmospheric pressure = -0.5ºC

1kg of propane


1 Watt is 1 Joule per second

Physical Properties of Water

Density = 1000kgm-3

Specific Heat Capacity of Water (Quantity of heat to increase temperature by 1oC = 4.182 kJ/kg


Power = Current x Volts (e.g. 10A current at 240 V = 2400W or 2.4kW)

Current = Power /Volts (e.g. 2.4kW at 240V draws a current of 10A)