Kingspan electric water heatersGetting the right size hot water cylinder is one of the most common concerns of our customers. This applies to people looking to install or replace a ‘whole house’ water heater, often referred to as either an ‘immersion heater’, a ‘hot water tank’, ‘hot water cylinder’ or ‘water boiler’.

The amount of readily available litres of hot water required will depend upon many factors, including:



  • the size of the household
  • if baths are involved
  • if showers are reliant on this stored water
  • how many sinks and hand basins are used
  • temperature preferences
  • work patterns
  • times of high demand
  • recovery times
  • and even things like the length of the pipe runs and how long certain members of the household like to spend under the shower!

No two households would necessarily be the same of course.Electric water cylinders

You could spend a lot of time and effort to get the right size hot water cylinder for your particular application based on each member of the household, and their individual needs based on their preferences and their habits, but for this article, let’s keep it simple.

Here are some guidelines to help you do some straightforward calculations:

  • A bath uses approximately 100L @ 40 degrees C
  • An average shower uses around 35L @ 40 degrees C (5 mins @ 7L per min)
  • Maybe allow 10 – 15L for a kitchen/utility sink
  • 5L or so will be sufficient for a wash hand basin

(We don’t really need to consider things like dishwashers or washing machines, as modern appliances tend to be cold feed only)

Of course, not all these things would be used at the same time, but it would be a good idea to consider the household in terms of consecutive showers in the morning, or perhaps in some cases, consecutive baths.

The other important factor to remember is that a typical temperature preference for hot water to be maintained in the cylinder would be around 60 or 65 degrees Celsius. So, for example, you wouldn’t need 100L of water at this temperature for a bath – perhaps only 60 or 70L when mixed with the cold water, and a similar ratio for a shower.

So, using the suggestions above, we’ll look at the right size hot water cylinder for an example household.

2 adults, 2 children, 1 bath, 2 showers, 2 wash hand basins, 1 kitchen sink.

  • Assume just one bath, not consecutive ones: 70L
  • Assume a possibility of 3 consecutive showers: 75L
  • Add on some basin and sink use at a similar time: 20L

(Remember we only really need to calculate the presumed ‘highest demand scenario’ and allow for blending with cold water. The hot water will of course re-heat or recover during less demanding periods throughout the day).

So in the example above, the total required litres at a peak time could be around 165.heatrae sadia megaflo

Popular models of hot water cylinders suitable to supply entire dwellings range from 90L up to 300L.

Perhaps, for the household in this illustration, a good choice could be the Heatrae Sadia Megaflo Eco 170DD   (170L, 6kw) or maybe a Kingspan Albion Ultrasteel AUD180  (180L 6kw)

Although we don’t need to get into kilowatts in any detail here, the simple rule is: The higher the Kw, the faster the recovery time. (Most smaller capacity water heaters tend to be 3kw, and most above 100L will be 6kw and upwards)

Here is a useful article regarding typical recovery times.

Sometimes, unfortunately, available space (generally height) could determine the maximum size tank you could go for, although some manufacturers now have horizontal versions available.

So there you have it – a simple, quick way to calculate the size of the hot water cylinder to suit your household – more info or help is always available from one of our water heating experts. Just call 01934 253600.