Heating & hot water
Energy to heat stored water
- where
- energy (J; the calculator shows kWh)
- fluid density (kg/m³)
- stored water volume (m³; any unit you pick is converted)
- specific heat capacity (J/(kg·K); any unit you pick is converted)
- final water temperature (°C)
- starting water temperature (°C)
Explain this equation
Works out the energy needed to heat a stored volume of water from one temperature to another. Use it for cylinder heat-up energy or running cost estimates.
ρ × V is the mass of water heated. c_p is the energy to warm one kilogram by one degree. T_h − T_c is the temperature rise from starting to final temperature.
Energy is mass × specific heat × temperature rise, the basic sensible heat relationship. The calculator works in joules and divides by 3.6 million to show kWh.
Sense check: 150 L from 10 °C to 60 °C needs about 8.7 kWh. A useful rule is 1.16 Wh per litre per degree. Double the volume or the rise and the energy doubles.
Watch out: this is ideal heating of water alone. Cylinder losses, heating the metal and draw-off during heating are not included, so real energy use is higher. The final temperature must be above the start.
Enter your values to see the result.
Good to know
Ideal sensible heating of water only. Cylinder metal, heat losses and draw-off are excluded; water must remain liquid.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.