Sizing a hot water cylinder

Sizing a hot water cylinder need not be complicated. If the cylinder is correctly sized from the outset there will be no risk of running out of hot water and having to wait for the cylinder to heat up again.

The easiest way to remember sizing cylinder’s is the larger the house or flat, the larger the cylinder required. You should always allow between 35 and 45 litres for every resident in the household. It is however, difficult to generalise when personal habits, and lifestyle plays a large factor in total hot water usage. Always consult the resident’s in the household when choosing a cylinder size and consider the following:

  • Showers use in the region of 18 litres of hot water per minute at 40 degrees Celsius (this equals 11 litres at 60 degrees Celsius)
  • Bath’s use in the region of 100 litres of hot water at 40 degrees Celsius (this equals 60 litres at 60 degrees Celsius)


The following three hot water consumption categories can be used as a guideline of how much hot water a person uses a day. The estimated water consumption daily per person can then be multiplied by how many residents are in the household to give you a daily hot water total usage:

Low consumption: 20-30 litres

Average consumption: 30-50 litres

High consumption: 50-70 litres

On these calculations a standard four-person household would require a minimum of a 200-litre hot water tank. It is this process of calculating hot water consumption which is used by the EU energy labelling scheme. It is ultimately the installers responsibility to calculate the correct size tank required to prevent the residents running out of hot water.

If in any doubt, we would always advise you to contact a professional when sizing a hot water cylinder. For further help and support please contact us.

Sales Service & Repair of Electric Thermal Stores.