Work out hot water cylinder sizing instantly with clear inputs, formula shown and shareable results.
Cylinder volume is daily demand per person multiplied by occupancy. Reheat energy is the familiar m x c x ΔT with water at 4.18 kJ/kg·K, converted to kilowatt hours by dividing by 3600. Because stored water at 60 °C is blended down at the tap, the usable volume at 40 °C is larger than the cylinder itself.
Reheat energy
kWh = litres x 4.18 x ΔT / 3600
Blended volume
Usable at 40 °C = volume x (store - cold) / (40 - cold)
kWh = litres x 4.18 x ΔT / 3600. Cylinder volume is daily demand per person multiplied by occupancy.
Storage above 60 °C controls legionella. The water is then blended at outlets by a thermostatic valve so the delivered temperature stays safe to touch.
This calculator takes 4 inputs: Occupants, Hot water per person per day, Store temperature, Cold feed temperature. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.